The "Packaging projects" tutorial should discuss using pyproject.toml

[pfmoore]

The tutorial at https://packaging.python.org/tutorials/packaging-projects/ describes building an old-style setuptools project. We should be covering using pyproject.toml. Reasons include:

1. It's the "modern" way of defining packages.
2. It ensures that the project defines its build dependency on wheel, which avoids oddities such as pip doing a direct setup.py install when installing from source.

See pypa/setuptools#510 (comment) and pypa/pip#8102 for context.

[bhrutledge]

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