Using black as an autoformatter

[tkrabel-db]

If I understand correctly, then we don't use any autoformatter in this repo (pls correct me if I'm wrong). If that is the case, I propose to use black as the formatter and to enforce it when submitting PRs.

I am happy to open a PR that formats all files if you want. Also happy to add the autoformatting to submitted PRs. WDYT @ccordoba12 / @krassowski ?