Use Github actions for PyPI publication

So far I've been doing PyPI publication by running `poetry publish` myself.

It would be nice to be able to push a tag (or an already tagged commit) to `main` and have that automatically kick off a build and publication.

For bonus points, we could even autogenerate the `CHANGELOG.md`, though so far its entries haven't been verbatim based on merge commit messages, so if we do that we'll need to be extra attentive to PR titles and merge commit messages. #33 might need to be implemented first.