.gitlab-ci.yml: Test on Python 3.9 and 3.11
Drop 3.8 testing as Sphinx does not support it anymore and will now be stuck at an old version, so we cannot have a stable man page generation that provides the same result across all builds.
Drop 3.8 testing as Sphinx does not support it anymore and will now be stuck at an old version, so we cannot have a stable man page generation that provides the same result across all builds.