[CI] Add `pyproject.toml` for `black`

[eddiebergman]

When starting up my editor, black wanted to format to 88. Upon looking, there's no specification anywhere. Turns out it should be in a pyproject.toml file.

In general, we can migrate most tools over to a pyproject.toml but not really important to do now

[PGijsbers]

I agree. This project pre-dates pyproject.toml (at least the support for it by various tools), but I would very much like to move as much of the different tool configurations in there, so everything is configured in an easily identifiable central place.