Migrate to pyproject.toml

[matthewcarbone]

I think it's PEP 621 that recommends we begin to migrate package metadata to the pyproject.toml file instead of a combination of setup.py, setup.cfg and other metadata files. I'm happy to take a shot at doing this if desired. I believe I know how (at the very least, I have a way to do it, which can be built upon). I can also provide a tutorial on how to couple it with GitHub actions (there is an open PR for this #105, but it appears to be quite stale). Let me know (I want to be sure there's still interest in making significant updates to this repo before I do)!