Publish to PyPI using GitHub Actions

The creation of a release in GitHub could be used to also publish a new release to PyPI. Either add a new job to the existing Actions flow or create a new flow all together. The credentials should be available from `astropy` organization.

Start by reviewing the existing GitHub Actions flow and how releases are created in GitHub. Determine where publishing belongs and how the astropy organization credentials are exposed; done means creating a GitHub release also publishes the corresponding package to PyPI.

Written by the indexing model from the issue text.