Consolidate GHA pipeline (cross-platform tests, PyPI releases)

[bittner]

The current GHA workflows for tests are split up between "Linux + macOS" and "Windows", and there is no workflow for releasing to PyPI.

Would you be interested in a PR to consolidate the unit test execution for all architectures (example) and add a release workflow that reacts on Git tags (example)?

Implementation note: In order to make the test execution cross-platform it's possible to move the prerequisite installation to a script and call the script(s) from the YAML configuration. (Explanation/StackOverflow)

This might also address/fix issue #146 as a side-effect.