Move black configuration to pyproject.toml

[mikkonie]

It seems Django auto-formats migrations with Black now. However, it won't use our proper configuration as that only comes from the Makefile.

The options should instead be moved into pyproject.toml to enforce similar formatting everywhere.

[mikkonie]

Postponing this for now, as flake8 and pycodestyle don't seem to support pyproject.toml? It would be nicer to have these in the same config than creating multiple files.. I'll think about it for v1.1.