Should use pyproject.toml or setup.cfg instead of setup.py

[smheidrich]

Now that PyO3/setuptools-rust#208 has been implemented in setuptools-rust, there is no longer any reason to use setup.py instead of pyproject.toml or setup.cfg => replace it.

[smheidrich]

Will require support in https://github.com/smheidrich/postprocess-sdist-make-rust-ext-opt first (and that thing also finally has a solid necessity).