Add PyPi publishing workflow

[fsimonis]

This PR adds a workflow that automatically publishes pushed tags v* to PyPi.

Important

You need to generate a PyPi token and add it as a secret called PYPI_TOKEN to the repo or organisation. If you add it to the org, make sure to expose it to this repo.

After that, creating and pushing a new tag will release the current state.

[imirzov]

Dear Frederic, I've added you as a maintainer to this repo. Please, could you generate PYPI_TOKEN by yourself? I'm slightly busy those weeks. Let me know, if you need more permissions.

[fsimonis]

Hi there,
generating and setting up the token also requires access to PyPi, its 2FA etc.
I gladly leave this to you for whenever you have time. 👍