Blacken codebase + add pre-commit hook + lint in CI

[jorisvandenbossche]

Closes #107

[jorisvandenbossche]

I added black and flake8 to the pre-commit hook, and now also a github action reuses this pre-commit hook as check on CI.

[choldgraf]

I'll give it until tomorrow morning for somebody to suggest otherwise, then I'll merge. I think it's a good idea to merge sooner than later, since if we don't merge relatively quickly we're going to hit merge conflicts each time the codebase changes.