Drop support for Python < 3.10 and add support for Python 3.12

Supporting the last three Python 3 releases (at the moment: 3.10, 3.11 and 3.12) should be good enough for Krake and means less maintainance effort, especially with respect to dependency management and pinning. For example, some of the packages we (indirectly) depend on do not support Python 3.8 anymore and are stuck at an older version. Furthermore, older Pythons require a lot of backported packages which are nowadays in the stdlib.

Checklist (no guarantee for completeness):

• Update README and other documentation
• Update images used in GitLab CI pipeline
• Update base image in Dockerfiles
• Update pyproject.toml files
• Update tox.ini

(Remark: this should wait until !556 (merged) is merged.)