Automate the release process with github actions

[tristanlatr]

Having a job that does the pypi release is really helpful to ensure the process stays the same on the long run and no one forgets to tag or to remove working files.

Here is an example of implementation: https://github.com/twisted/pydoctor/blob/master/.github/workflows/unit.yaml#L72

It will release every time a new tag is pushed and the CI is green.