Add publishing pipeline

[SCiarella]

This pull request addresses #27 by implementing a publishing pipeline in .github/workflow/publish.yaml.
The action is triggered every time a new version of lie_learn gets published and it automatically updates the entry on pypi.
In this way, downstream libraries (like escnn) can use lie_learn much more easily.

To fully close #27 it is required to:

1. enable trusted publishing
2. either publish a new release of lie_learn or manually trigger publish.yaml via:
   Actions->'Build and Publish'->Run Workflow

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Due to the end of life the current process only supports python>=3.9, but it is possible to include older versions to this workflow if needed.

Finally, notice that there are some issues with the recent release of Numpy 2.0.0 (also discussed in #30), so we have enforced numpy<2 in the dependency.

[tscohen]

LGTM. @mariogeiger and @bkmi has are maintaining the PyPi package, so only they can enable trusted publishing. I'll send them a message as well.