contrib/python-zstandard/MANIFEST.in

hook: add a 'hashook' function to test for hook existence

Preparing the data for some hooks can be expensive. Add a function to check if
a hook exists so we can skip useless preparation if no hook is configured.

graft c-ext
graft zstd
include make_cffi.py
include setup_zstd.py
include zstd.c