Adopt GitHub Actions for CI

[cmacmackin]

This contains the configuration file for GitLab CI but obviously that won't work now you're on GitHub. Instead use GitHub Actions. These have some very powerful features, allowing you to reuse other people's configurations across repositories. For an example of their use in a Python code, see the InDiCA project.

[P-S-S-M]

All gitlab workflows running, including the deployment to PyPI. Should be good to close issue