Python 3.8 was removed from the tested runtimes for 2024.2[1] and has
not been tested since then.
Also add Python 3.12 which is part of the tested runtimes for 2025.1.
Now unit tests job with Python 3.12 is voting.
[1] https://governance.openstack.org/tc/reference/runtimes/2024.2.html
Change-Id: Ifc461fd0eccde637f55c24610f1f81d51cab1689