Transition to pyproject.toml

[jacobdadams]

The current preferred way of controlling python packaging is through a pyproject.toml file that can mostly or entirely replace setup.py. This is a note to get this on our radar and save progress/knowledge.

This may allow us to stop tracking the version number in source code completely, relying on github tags if we want.

References:
https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html

[stdavis]

This is done.