[Housekeeping] Formatting all code with black

Hi all, Cosimo and me would like to format all code with https://github.com/psf/black to unify style and automate the boring tasks of fitting your code into the surrounding style. This would also include a CI lint check to make sure the black style is adhered to for any incoming code.

Two known difficulties:

1. Existing PRs may need to be rebased on top of the newly formatted code.
2. Making git blame work as before requires an extra step: https://github.com/psf/black#migrating-your-code-style-without-ruining-git-blame

Thoughts?

[justvanrossum]

Ok, fair enough. And as a side effect we’ll lose all tabs, yes? That’d be worth a party.

[behdad]

I mean, PRs will keep coming forever I suppose and there are very old ones.

I’m going through the old ones and clearing them up is also a very important housekeeping task. They’re mostly stale already 😦.