Use black to standardize code formatting

as per https://black.readthedocs.io/ .

I just ran black on https://github.com/fnielsen/scholia/commit/8f37b606192840dabbee67f50f39e21a94f47c8a#diff-429257c2eb8f90b387787b4cdd724973d9bc3a1fc0fddee5b9877077a2de1e6e and got

```28 files reformatted, 2 files left unchanged.```

This could also be included in our CI checks.