Reinstate auto-publishing to PyPi

[stevejalim]

The TravisCI integration used to auto-publish to PyPi when a new version was merged to master. However, this had stopped working and when we moved to GH Actions, it was not reinstated. We should do that, with GH Actions

Success criteria

• Merging to master will trigger a new release on PyPi, if the relevant version does not exist in PyPi already

[janbrasna]

This was added in #87 — just no release has been published since to verify;) Do you want to keep the issue around open as a reminder when a GH Release is published to verify the automation and API tokens et al., or it can be closed as done?

[janbrasna]

FYI you can now get rid of the PYPI_API_TOKEN and switch to "Trusted Publishing" (OIDC) if you upgrade the pypa action to v1.8.x+ or v1.9: https://docs.github.com/en/actions/security-for-github-actions/security-hardening-your-deployments/configuring-openid-connect-in-pypi