Consider the adoption of Black-style code formatting and tooling

[nicolaiarocci]

I like Black.

Two small concerns:

• Unsure on the 88 chars line-length. Can be trimmed down to 79/80, but it will still leave unbreakable lines (long strings, for example) untouched. Overall I think the 10% rule is good, and I could also live with longer lines when breaking would make them basically unreadable (long strings, again).
• Landing a successful PR might become more tedious. With version control integration and a small addendum to the contributing instructions, we might make this more manageable. This is probably not a concern since we already run flake8 on CI - we'd only need to tweak it a little bit (flake8-bugbear, etc.)

[nicolaiarocci]

We might also want to hold until Black reaches beta. Should not take too long.

[nicolaiarocci]

Black is now in beta: https://twitter.com/llanga/status/1001411218987302912