Adapt to new build concept

• use setuptools_scm to define the version based on git tags
• use setup.cfg instead of setup.py where possible
• use pyproject.toml
• use importlib.metadata to read the version in the code
• don't use alpine docker builds as they do not supply a dev environment
• don't add _version.py to git (autogenerated by setuptools_scm)
• push build to pypi