contrib/python-zstandard/MANIFEST.in

makefile: use Python 3 by default when building docs as well

I copied the snippet defining `PYTHON` from the root Makefile.

Differential Revision: https://phab.mercurial-scm.org/D10251

graft c-ext
graft debian
graft zstd
graft tests
include make_cffi.py
include setup_zstd.py
include zstd.c
include LICENSE
include NEWS.rst