v4.0: Remove `setup.py` in favor of `pyproject.toml`?

[ajfriend]

Is it possible to drop setup.py for the more-modern pyproject.toml approach?

https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/

Dropping legacy python versions in v4.0 should help unlock this.

[kylebarron]

The main blocker here is scikit-build, and that doesn't appear to have any documentation for working with pyproject.toml, except in conjunction with setup.py