Related to T2610

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

This is a temporary move.  This will eventually converge with
swh.model.cli as the directory hashing ability overlaps.  What's
missing in the cli for now is the ability to filter patterns.

This is mostly used to compute hashes on tree coming from multiple
dvcs. This imposes the need to be able to filter pattern like .svn,
.hg, etc...

Currently only content and directory object types are supported, but more can
be added in the future.

Closes T1039

reincarnation of the old shw.model bin/ script, which is now gone