Replace setup.py mentions in "Packaging and distributing projects" guide with pyproject.toml, or merge guide with tutorial

[astrojuanlu]

This guide https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/ still uses setup.py files, and in fact it doesn't actually mention pyproject.toml files at all.

I wonder though if, instead of updating the guide, it would just be better to remove it or merge it with the more up to date tutorial https://packaging.python.org/en/latest/tutorials/packaging-projects/ which was already updated as per gh-1084.

[bhrutledge]

I think this is a duplicate #887 and #792.