migrate to black

[utnapischtim]

• migrate to use black as opinionated auto formater
• add .git-blame-ignore-revs
• fix docs compatibilty problem with Sphinx>=5.0.0
• move check_manifest configuration to setup.cfg.
• increase minimal python version to 3.7
• global: clean test infrastructure

[max-moser]

i left a few comments, which serve as either questions to @utnapischtim or markers for other reviewers so they don't have to wade through all the formatting changes, because this PR is quite big

to summarize:

• code was formatted with black
• a few configs were changed to interfere less with black
• code was made compatible with flask-login 0.5+ (user_id -> _user_id)
• some test functions got parametrized (not only because it's fancy but also because it wasn't easily possible to change the logged-in user within the function)
• the test matrix was updated

anything missing?