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.