CI: add automatic publication to pypi.org and test.pypi.org

[hnez]

Publishing software manually is a (somewhat) error-prone and (somewhat) tedious process, hence why other projects have already automated this process.

Let's join these other projects and also automate our publication process!

The setup process is surprisingly straight forward and involves setting up trusted publishing on both pypi.org, as well as test.pypi.org for the lxa-iobus project:

And setting a GitHub Action Variable that enables publication (so that the job does not run and fail for forked repositories by default):

TODO before merging:

• [x] This PR is based on #44, which enables automatic version number generation from git tags
• [x] and PR #45 which prepares the CI jobs for this PR.

Both of these PRs should be merged before this one.

[hnez]

I should mention that this PR is heavily inspired by @jluebbe's work over at pdudaemon/pdudaemon#155. I did however tweak a few aspects:

• Use an if-block at the job level to prevent the job from even starting when the conditions are not met.
• Use a GitHub action variable to prevent the job from running for forks.
• Always publish to test.pypi.org, even for tags.