Related to T2610

Related to T2105

Related to T2367

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

'swh' being the new master command group (defined in swh.core); so:

  swh deposit --help

works. The 'admin' (deposit server management) is only available if the
swh.deposit[server] optional package is installed.

Ensure the 'swh-deposit' still works for now.

tests_require != test_requires

related to T1242

This does the dependencies declaration the same way the
python3-swh.lister package does it.

Related T882

Goal: Reducing dependencies

WIP, this still needs:
- integration testing (manual so far)
- clean up routine
- scheduler api interface (for now this uses celery directly)

Also remove unused dependencies