Sphinx version support strategy and CI update

[danwos]

There were some discussions in the team, about which Sphinx version and therefore Python to support. This is the outcome and your CI and Tests needs to get updated related to this:

Sphinx: We support the latest 3 MAJOR versions, so currently 5,6,7. Python: We support the Python versions requested by supported Sphinx versions. So currently Python 3.7 (from Sphinx 5.x) and the latest officially released Python version.

Tasks:

• [ ] Update contribution-docs
• [ ] Update test cases to support each version
• [ ] Update code to support deprecated and newer APIs

For Sphinx 7.x, there is already #1045, but it drops support for older versions and can't therefore be merged.