Update PEP 561 examples with modern packaging

[ofek]

Documentation

https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages as noted in https://github.com/pypa/hatch/discussions/554 only mentions setup.py which is deprecated for simply defining metadata, see PEP 621

It would be nice to mention the Hatchling way as that is now the default in the official Python packaging tutorial.

[ssbarnea]

Anyone against me making a patch to migrate packaging to PEP-621 with setuptools-scm?

[ofek]

Here's an example that uses setuptools-scm and Mypyc https://github.com/psf/black/blob/main/pyproject.toml

[hauntsaninja]

Thanks, that'd be a welcome addition! However, I'd still like to keep the setup.py example in the docs (though it could also use some modernisation e.g. no distutils)