It seems that setup.py gets deleted before python executes it in
clean-local. To prevent this situation, we just use .NOTPARALLEL.
Previously we distribute nghttp2.c in python, which is cythonized C
source code from nghttp2.pyx. Since it is distributed in archive it
exists in source directory. But we use python distutils which
operates in build directory and does not support C source files in out
of tree directory (i.e., not under the build directory). Copying C
source file to build directory is a bit dirty, so we just decided not
to ship nghttp2.c.
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