CI: Add continuous delivery (packaging and uploading to PyPI)

Michael Thiesrequested to merge
feature/continuous_delivery into master

This MR adds the new stages package and upload to the CI pipeline. The upload job is only performed for Version-Tags (vX.X.X), which are protected in GitLab to be only created by Maintainers.

Additionally, this MR fixes the setup.py to exclude the tests from the distribution package.

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