[3.6] bpo-31221: patchcheck ignores external libraries (#3109) (#3116)

[3.6] bpo-31221: patchcheck ignores external libraries (#3109) (#3116)

* bpo-31221: patchcheck ignores external libraries (#3109)

Tools/scripts/patchcheck.py now ignores changes in directories which
are copies of external libraries:

* Modules/_ctypes/libffi_msvc/
* Modules/_ctypes/libffi_osx/
* Modules/_decimal/libmpdec/
* Modules/expat/
* Modules/zlib/

Drop also support for Mercurial, since CPython migrated to Git.

Exclude also libmpdec

patchcheck: exclude also libffi_osx and libffi_msvc
(cherry picked from commit 4a347ce426fe7381885703d9074d7a6b3aeb2f2b)

* Exclude also Modules/_ctypes/libffi on Python 3.6