Move all setup.cfg configuration into pyproject.toml

[akx]

Mypy can read its configuration just fine from pyproject.toml too, there's no reason to keep the legacy setup.cfg file hanging on.

This also adds the build-system meta for setuptools so the package can be built using regular PEP 517 tooling (e.g. python -m build or uv build).

I checked the built distributions are still the same c.f. 0d69b77 (aside, of course, these changed files).

[attwad]

thanks!