diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index d45e3a0b..589e466d 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -8,7 +8,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-10.15]
+        os: [ubuntu-20.04, macos-10.15]
         compiler: [gcc, clang]
         buildtool: [autotools, cmake]
 
diff --git a/README.rst b/README.rst
index 87f6ce4f..c5c2eaa7 100644
--- a/README.rst
+++ b/README.rst
@@ -112,7 +112,7 @@ libnghttp2_asio C++ library requires the following packages:
 The Python bindings require the following packages:
 
 * cython >= 0.19
-* python >= 2.7
+* python >= 3.8
 * python-setuptools
 
 If you are using Ubuntu 16.04 LTS (Xenial Xerus) or Debian 8 (jessie)
@@ -1422,7 +1422,7 @@ The extension module is called ``nghttp2``.
 determined by the ``configure`` script.  If the detected Python version is not
 what you expect, specify a path to Python executable in a ``PYTHON``
 variable as an argument to configure script (e.g., ``./configure
-PYTHON=/usr/bin/python3.5``).
+PYTHON=/usr/bin/python3.8``).
 
 The following example code illustrates basic usage of the HPACK compressor
 and decompressor in Python:
@@ -1494,7 +1494,7 @@ BaseRequestHandler usage:
 
 .. code-block:: python
 
-    #!/usr/bin/env python
+    #!/usr/bin/env python3
 
     import io, ssl
     import nghttp2
diff --git a/configure.ac b/configure.ac
index 001b1abd..a26b4d81 100644
--- a/configure.ac
+++ b/configure.ac
@@ -151,7 +151,7 @@ AC_PROG_MKDIR_P
 
 PKG_PROG_PKG_CONFIG([0.20])
 
-AM_PATH_PYTHON([2.7],, [:])
+AM_PATH_PYTHON([3.8],, [:])
 
 if [test "x$request_lib_only" = "xyes"]; then
   request_app=no
@@ -161,7 +161,7 @@ if [test "x$request_lib_only" = "xyes"]; then
 fi
 
 if [test "x$request_python_bindings" != "xno"]; then
-  AX_PYTHON_DEVEL([>= '2.7'])
+  AX_PYTHON_DEVEL([>= '3.8'])
 fi
 
 if test "x${cython_path}" = "x"; then
@@ -567,7 +567,7 @@ enable_python_bindings=no
 if test "x${request_python_bindings}" != "xno" &&
    test "x${CYTHON}" != "x" &&
    test "x${PYTHON}" != "x:" &&
-   test "x${have_python_dev}" = "xyes"; then
+   test "x${PYTHON_VERSION}" != "x"; then
   enable_python_bindings=yes
 fi
 
@@ -890,9 +890,8 @@ AC_MSG_NOTICE([summary of build options:
       Python:         ${PYTHON}
       PYTHON_VERSION: ${PYTHON_VERSION}
       pyexecdir:      ${pyexecdir}
-      Python-dev:     ${have_python_dev}
       PYTHON_CPPFLAGS:${PYTHON_CPPFLAGS}
-      PYTHON_LDFLAGS: ${PYTHON_LDFLAGS}
+      PYTHON_LIBS:    ${PYTHON_LIBS}
       Cython:         ${CYTHON}
     Test:
       CUnit:          ${have_cunit} (CFLAGS='${CUNIT_CFLAGS}' LIBS='${CUNIT_LIBS}')
diff --git a/doc/sources/python-apiref.rst b/doc/sources/python-apiref.rst
index d64b43cc..92fab33f 100644
--- a/doc/sources/python-apiref.rst
+++ b/doc/sources/python-apiref.rst
@@ -13,7 +13,7 @@ The extension module is called ``nghttp2``.
 determined by configure script.  If the detected Python version is not
 what you expect, specify a path to Python executable in ``PYTHON``
 variable as an argument to configure script (e.g., ``./configure
-PYTHON=/usr/bin/python3.5``).
+PYTHON=/usr/bin/python3.8``).
 
 HPACK API
 ---------
@@ -137,14 +137,14 @@ HTTP/2 servers
 .. note::
 
    We use :py:mod:`asyncio` for HTTP/2 server classes, and ALPN.
-   Therefore, Python 3.5 or later is required to use these objects.
-   To explicitly configure nghttp2 build to use Python 3.5, specify
-   the ``PYTHON`` variable to the path to Python 3.5 executable when
+   Therefore, Python 3.8 or later is required to use these objects.
+   To explicitly configure nghttp2 build to use Python 3.8, specify
+   the ``PYTHON`` variable to the path to Python 3.8 executable when
    invoking configure script like this:
 
    .. code-block:: text
 
-       $ ./configure PYTHON=/usr/bin/python3.5
+       $ ./configure PYTHON=/usr/bin/python3.8
 
 .. py:class:: HTTP2Server(address, RequestHandlerClass, ssl=None)
 
@@ -336,7 +336,7 @@ The following example illustrates :py:class:`HTTP2Server` and
 
 .. code-block:: python
 
-    #!/usr/bin/env python
+    #!/usr/bin/env python3
 
     import io, ssl
 
@@ -367,7 +367,7 @@ response body generation.  This is simplified reverse proxy:
 
 .. code-block:: python
 
-    #!/usr/bin/env python
+    #!/usr/bin/env python3
 
     import ssl
     import os
diff --git a/m4/ax_python_devel.m4 b/m4/ax_python_devel.m4
index 067fbcf3..44dbd83e 100644
--- a/m4/ax_python_devel.m4
+++ b/m4/ax_python_devel.m4
@@ -1,5 +1,5 @@
 # ===========================================================================
-#      http://www.gnu.org/software/autoconf-archive/ax_python_devel.html
+#     https://www.gnu.org/software/autoconf-archive/ax_python_devel.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -12,8 +12,8 @@
 #   in your configure.ac.
 #
 #   This macro checks for Python and tries to get the include path to
-#   'Python.h'. It provides the $(PYTHON_CPPFLAGS) and $(PYTHON_LDFLAGS)
-#   output variables. It also exports $(PYTHON_EXTRA_LIBS) and
+#   'Python.h'. It provides the $(PYTHON_CPPFLAGS) and $(PYTHON_LIBS) output
+#   variables. It also exports $(PYTHON_EXTRA_LIBS) and
 #   $(PYTHON_EXTRA_LDFLAGS) for embedding Python in your code.
 #
 #   You can search for some particular version of Python by passing a
@@ -52,7 +52,7 @@
 #   Public License for more details.
 #
 #   You should have received a copy of the GNU General Public License along
-#   with this program. If not, see <http://www.gnu.org/licenses/>.
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
 #
 #   As a special exception, the respective Autoconf Macro's copyright owner
 #   gives unlimited permission to copy, distribute and modify the configure
@@ -67,7 +67,7 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 16
+#serial 21
 
 AU_ALIAS([AC_PYTHON_DEVEL], [AX_PYTHON_DEVEL])
 AC_DEFUN([AX_PYTHON_DEVEL],[
@@ -81,12 +81,10 @@ AC_DEFUN([AX_PYTHON_DEVEL],[
 
 	AC_PATH_PROG([PYTHON],[python[$PYTHON_VERSION]])
 	if test -z "$PYTHON"; then
-	   AC_MSG_WARN([Cannot find python$PYTHON_VERSION in your system path])
+	   AC_MSG_ERROR([Cannot find python$PYTHON_VERSION in your system path])
 	   PYTHON_VERSION=""
-           no_python_devel=yes
 	fi
 
-AS_IF([test -z "$no_python_devel"], [
 	#
 	# Check for a version of Python >= 2.1.0
 	#
@@ -97,25 +95,22 @@ AS_IF([test -z "$no_python_devel"], [
 	if test "$ac_supports_python_ver" != "True"; then
 		if test -z "$PYTHON_NOVERSIONCHECK"; then
 			AC_MSG_RESULT([no])
-			AC_MSG_WARN([
+			AC_MSG_FAILURE([
 This version of the AC@&t@_PYTHON_DEVEL macro
 doesn't work properly with versions of Python before
 2.1.0. You may need to re-run configure, setting the
-variables PYTHON_CPPFLAGS, PYTHON_LDFLAGS, PYTHON_SITE_PKG,
+variables PYTHON_CPPFLAGS, PYTHON_LIBS, PYTHON_SITE_PKG,
 PYTHON_EXTRA_LIBS and PYTHON_EXTRA_LDFLAGS by hand.
 Moreover, to disable this check, set PYTHON_NOVERSIONCHECK
 to something else than an empty string.
 ])
-                        no_python_devel=yes
 		else
 			AC_MSG_RESULT([skip at user request])
 		fi
 	else
 		AC_MSG_RESULT([yes])
 	fi
-]) # AS_IF
 
-AS_IF([test -z "$no_python_devel"], [
 	#
 	# if the macro parameter ``version'' is set, honour it
 	#
@@ -128,36 +123,30 @@ AS_IF([test -z "$no_python_devel"], [
 		   AC_MSG_RESULT([yes])
 		else
 			AC_MSG_RESULT([no])
-			AC_MSG_WARN([this package requires Python $1.
+			AC_MSG_ERROR([this package requires Python $1.
 If you have it installed, but it isn't the default Python
 interpreter in your system path, please pass the PYTHON_VERSION
 variable to configure. See ``configure --help'' for reference.
 ])
 			PYTHON_VERSION=""
-                        no_python_devel=yes
 		fi
 	fi
-]) # AS_IF
 
-AS_IF([test -z "$no_python_devel"], [
 	#
 	# Check if you have distutils, else fail
 	#
 	AC_MSG_CHECKING([for the distutils Python package])
 	ac_distutils_result=`$PYTHON -c "import distutils" 2>&1`
-	if test -z "$ac_distutils_result"; then
+	if test $? -eq 0; then
 		AC_MSG_RESULT([yes])
 	else
 		AC_MSG_RESULT([no])
-		AC_MSG_WARN([cannot import Python module "distutils".
+		AC_MSG_ERROR([cannot import Python module "distutils".
 Please check your Python installation. The error was:
 $ac_distutils_result])
 		PYTHON_VERSION=""
-                no_python_devel=yes
 	fi
-]) # AS_IF
 
-AS_IF([test -z "$no_python_devel"], [
 	#
 	# Check for Python include path
 	#
@@ -183,7 +172,7 @@ AS_IF([test -z "$no_python_devel"], [
 	# Check for Python library path
 	#
 	AC_MSG_CHECKING([for Python library path])
-	if test -z "$PYTHON_LDFLAGS"; then
+	if test -z "$PYTHON_LIBS"; then
 		# (makes two attempts to ensure we've got a version number
 		# from the interpreter)
 		ac_python_version=`cat<<EOD | $PYTHON -
@@ -238,29 +227,26 @@ EOD`
 		then
 			# use the official shared library
 			ac_python_library=`echo "$ac_python_library" | sed "s/^lib//"`
-			PYTHON_LDFLAGS="-L$ac_python_libdir -l$ac_python_library"
+			PYTHON_LIBS="-L$ac_python_libdir -l$ac_python_library"
 		else
 			# old way: use libpython from python_configdir
 			ac_python_libdir=`$PYTHON -c \
 			  "from distutils.sysconfig import get_python_lib as f; \
 			  import os; \
 			  print (os.path.join(f(plat_specific=1, standard_lib=1), 'config'));"`
-			PYTHON_LDFLAGS="-L$ac_python_libdir -lpython$ac_python_version"
+			PYTHON_LIBS="-L$ac_python_libdir -lpython$ac_python_version"
 		fi
 
-		if test -z "PYTHON_LDFLAGS"; then
-			AC_MSG_WARN([
+		if test -z "PYTHON_LIBS"; then
+			AC_MSG_ERROR([
   Cannot determine location of your Python DSO. Please check it was installed with
-  dynamic libraries enabled, or try setting PYTHON_LDFLAGS by hand.
+  dynamic libraries enabled, or try setting PYTHON_LIBS by hand.
 			])
-                        no_python_devel=yes
 		fi
 	fi
-	AC_MSG_RESULT([$PYTHON_LDFLAGS])
-	AC_SUBST([PYTHON_LDFLAGS])
-]) # AS_IF
+	AC_MSG_RESULT([$PYTHON_LIBS])
+	AC_SUBST([PYTHON_LIBS])
 
-AS_IF([test -z "$no_python_devel"], [
 	#
 	# Check for site packages
 	#
@@ -279,7 +265,7 @@ AS_IF([test -z "$no_python_devel"], [
 	if test -z "$PYTHON_EXTRA_LIBS"; then
 	   PYTHON_EXTRA_LIBS=`$PYTHON -c "import distutils.sysconfig; \
                 conf = distutils.sysconfig.get_config_var; \
-                print (conf('LIBS'))"`
+                print (conf('LIBS') + ' ' + conf('SYSLIBS'))"`
 	fi
 	AC_MSG_RESULT([$PYTHON_EXTRA_LIBS])
 	AC_SUBST(PYTHON_EXTRA_LIBS)
@@ -302,8 +288,10 @@ AS_IF([test -z "$no_python_devel"], [
 	AC_MSG_CHECKING([consistency of all components of python development environment])
 	# save current global flags
 	ac_save_LIBS="$LIBS"
+	ac_save_LDFLAGS="$LDFLAGS"
 	ac_save_CPPFLAGS="$CPPFLAGS"
-	LIBS="$ac_save_LIBS $PYTHON_LDFLAGS $PYTHON_EXTRA_LDFLAGS $PYTHON_EXTRA_LIBS"
+	LIBS="$ac_save_LIBS $PYTHON_LIBS $PYTHON_EXTRA_LIBS $PYTHON_EXTRA_LIBS"
+	LDFLAGS="$ac_save_LDFLAGS $PYTHON_EXTRA_LDFLAGS"
 	CPPFLAGS="$ac_save_CPPFLAGS $PYTHON_CPPFLAGS"
 	AC_LANG_PUSH([C])
 	AC_LINK_IFELSE([
@@ -314,15 +302,16 @@ AS_IF([test -z "$no_python_devel"], [
 	# turn back to default flags
 	CPPFLAGS="$ac_save_CPPFLAGS"
 	LIBS="$ac_save_LIBS"
+	LDFLAGS="$ac_save_LDFLAGS"
 
 	AC_MSG_RESULT([$pythonexists])
 
         if test ! "x$pythonexists" = "xyes"; then
-	   AC_MSG_WARN([
+	   AC_MSG_FAILURE([
   Could not link test program to Python. Maybe the main Python library has been
   installed in some non-standard library path. If so, pass it to configure,
-  via the LDFLAGS environment variable.
-  Example: ./configure LDFLAGS="-L/usr/non-standard-path/python/lib"
+  via the LIBS environment variable.
+  Example: ./configure LIBS="-L/usr/non-standard-path/python/lib"
   ============================================================================
    ERROR!
    You probably have to install the development version of the Python package
@@ -330,15 +319,9 @@ AS_IF([test -z "$no_python_devel"], [
   ============================================================================
 	   ])
 	  PYTHON_VERSION=""
-          no_python_devel=yes
 	fi
 
 	#
 	# all done!
 	#
-]) # AS_IF
-
-AS_IF([test -z "$no_python_devel"],
-      [have_python_dev=yes], [have_python_dev=no])
-
-]) # AS_IF
+])
diff --git a/python/Makefile.am b/python/Makefile.am
index 2d2edd5a..75eb7a05 100644
--- a/python/Makefile.am
+++ b/python/Makefile.am
@@ -44,6 +44,6 @@ clean-local:
 	-rm -f $(builddir)/nghttp2.c
 
 .pyx.c:
-	$(CYTHON) -o $@ $<
+	$(CYTHON) -3 -o $@ $<
 
 endif # ENABLE_PYTHON_BINDINGS
diff --git a/python/hpackcheck.py b/python/hpackcheck.py
index 64d2851d..41a8322e 100755
--- a/python/hpackcheck.py
+++ b/python/hpackcheck.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # This script reads json files given in the command-line (each file
 # must be written in the format described in
diff --git a/python/hpackmake.py b/python/hpackmake.py
index 208c0e1d..a96bbe58 100755
--- a/python/hpackmake.py
+++ b/python/hpackmake.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # This script reads input headers from json file given in the
 # command-line (each file must be written in the format described in
diff --git a/python/nghttp2.pyx b/python/nghttp2.pyx
index 7821ff14..c374d786 100644
--- a/python/nghttp2.pyx
+++ b/python/nghttp2.pyx
@@ -857,7 +857,7 @@ cdef class _HTTP2SessionCore(_HTTP2SessionCoreBase):
 
         rv = cnghttp2.nghttp2_submit_settings(self.session,
                                               cnghttp2.NGHTTP2_FLAG_NONE,
-                                              iv, sizeof(iv) / sizeof(iv[0]))
+                                              iv, sizeof(iv) // sizeof(iv[0]))
 
         if rv != 0:
             raise Exception('nghttp2_submit_settings failed: {}'.format\
@@ -971,7 +971,7 @@ cdef class _HTTP2ClientSessionCore(_HTTP2SessionCoreBase):
 
         rv = cnghttp2.nghttp2_submit_settings(self.session,
                                               cnghttp2.NGHTTP2_FLAG_NONE,
-                                              iv, sizeof(iv) / sizeof(iv[0]))
+                                              iv, sizeof(iv) // sizeof(iv[0]))
 
         if rv != 0:
             raise Exception('nghttp2_submit_settings failed: {}'.format\