Fix all Flake8 rules

[Glutexo]

Related to #189. This is a list of small subtasks and respective pull requests that would lead to a nicely formatted code. This code would

• stay unchanged when run through Black,
• not break any Flake8 rule that is not ignored with its default settings,
• be generally nice and consistent.

The concrete steps are:

1. Fix all Flake8 violations in reasonably small chunks. Formatting: fix Flake8 whitespace violations #334, Formatting: fix Flake8 indentation #336, Formatting: fix Flake8 newlines #337, Formatting: fix Flake8 imports #338, Formatting: move fixtures to conftest.py #341, Formatting: remove unused variables #342
2. Use Black to reformat everything that Flake8 doesn’t complain about. Formatting: unify quotes by Black’s standards #359 Formatting: unify newlines by Black’s standards #360 Formatting: contract multi-line expressions #361 Formatting: fix trailing commas #362 Formatting: fix whitespace by Black’s standards #363 Formatting: convert indent from visual to hanging #364
3. Manually go through all the code and reformat everything that slipped through. Formatting: string interpolation #365 Formatting: Simplify super calls #371

After that we’ll be ready to block any pull request that doesn’t conform to Flake8, Black and our standards. Pre-commit hooks (#331) could run non-destructively.