Update Packaging Python Projects tutorial to use PEP 518

[uranusjr]

Currently the tutorial instructs the user to construct the package without pyproject.toml. A package built as such however has some install-time perculirities that may confuse users (see pypa/pip#7709 pypa/pip#7716). Adding pyproject.toml into the initial tutorial would help nudge the user onto the right track from the beginning. It could also be helpful for driving PEP 518 adoption—or at least not hampering it by introducing more new non-PEP 518 packages.

[bhrutledge]

With the resolution of #809, the tutorial now includes a minimal pyproject.toml to use setuptools. Can this be closed?

Also, is this is a duplicate of #720?

[uranusjr]

Yeah, I think #809 covers this. The Packaging and distributing projects guide could also use an update, but that's a separate issue.