Switch to pyproject.toml entirely

[jeliebig]

This PR removes setup.py and instead uses pyproject.toml for everything, as setup.py is deprecated.

I adjusted and tested the workflows, so they still work with pyproject.toml.
(macOS doesn't have GNU sed, so I decided to split this step into two OS-specific ones, as I couldn't find a way to make the command work without breaking it on one of them.)

• I'll mark this PR as draft until release: 1.7.0 PyO3/setuptools-rust#355 is merged, so I can remove the git URL from build-system.requires again.

[theCapypara]

Thanks!