chore(ci): Use pytest-flake8 and pytest-black

[winged]

Instead of calling flake8 / black explicitly, add them to pytest. This
should help with forgetting to call those tools before committing, as
it's only one commandline call now instead of three.

[sliverc]

I see the benefit this could be bring, though not so sure about this change because of following reasons:

• performance: potentially black, flake8 are run too many times (with pytest, editor, pre-commit etc.) see also pytest-flake8 is very slow and uses lots of RAM with -vv tholo/pytest-flake8#19
• flake8 doesn't seem to honour default flake8 configuration which would be needed for editor (see Use of standard flake8 configuration locations tholo/pytest-flake8#22) - one of the reasons why CI is currently failing. - this only seems to be relevant for exclude option but this files could be made flake8 compatible as well so not really a problem.

I think it's kind of developer taste which way to go so would it be an option to document usage of pytest-flake8 and pytest-black in CONTRIBUTING.md and a developer can then choose whether to use its own editor, pre-commit or pytest plugins. What do you think?

Otherwise if we want to integrate per default into Caluma it would be good to get the feedback of other regular committers.

[winged]

Yeah makes sense