Add autopublish workflow

[ColCarroll]

This should allow a PyPI release when pyei.__version__ is greater than the latest version at https://pypi.org/project/pyei/. I also updated some of the github actions to more recent ones.

Note that this requires

1. creating a security token at https://pypi.org/manage/project/pyei/settings/, and then
2. going to https://github.com/mggg/ecological-inference/settings/secrets/actions and adding this key as a secret named "PYPI_API_TOKEN"

I think that's it! But it might require some futzing. In particular, the action only runs when you merge to main and update the version number, so we can't really tell if it worked until you merge this, do those two steps, and then bump the version number.

For what it is worth, both of bayeux and ridge_map use this action.

[karink520]

So helpful, thanks!