Modernize packaging tutorial

[henryiii]

The packaging tutorial contains quite a lot of dated suggestions; on browsing, I see at least the following things:

• pyproject.toml is missing
• setup.py is used for metadata instead of setup.cfg
• setup.py is run manually (python3 setup.py sdist bdist_wheel) instead of using PyPA-build

Would you like me to update the tutorial with these suggestions?

[JulienPalard]

This page looks the most up-to-date doc of how to build today: https://setuptools.readthedocs.io/en/latest/build_meta.html, which is what I recommend, so I'd personally find it nice to have the equivalent in the packaging tutorial, yes.