Code style with black

[marianheil]

Formating is getting anoying to do by hand. We should try again with black, similar to #71.

[jcwhitehead]

Is it possible/would it be sensible to automatically squash black commits into their parents?

I think my opinion is:

• flake is a useful check on the code
• it fails because of trivial formatting problems, as well as because of real problems
• we are therefore realistically either going to end up with 'flake fix' commits (my approach), or squash the fix into the original (your approach), neither is ideal and both take time
• best resolution is for black to fix the formatting as if we'd applied the rules perfectly in the first place, without generating messy additional commits in which nothing meaningful changes. This also allows blame to work.