Switch to pyproject.toml

[ipwnponies]

It seems the current way to build packages is using build and pyproject.toml. It's backwards compatible with setup.cfg.

We also want to migrate to from venv-update to poetry. Poetry uses pyproject.toml.

Let's rip the band-aid and get the work done, it seems a certainty that this is the de facto future direction.

[ipwnponies]

Closed in be90b6d