Migrate `setup.cfg` to `pyproject.toml`

[nathanjmcdougall]

This is basically cosmetic, but I think it tends to make things easier to keep everything in one place:

• Reduces the total number of configuration files
• Removes the possibility of configuration duplication/conflicts between the two files

Only blocker is #251 since flake8 cannot be configured within pyproject.toml.

[isabelizimm]

Agreed 💯 the less config files the better!