Support for python 3.13, drop 3.8 as EOL

[trappitsch]

It's this time of the year again: python 3.13 is out and 3.8 is officially EOL. Currently the tests dont succeed yet under 3.13. Waiting for pint to fix an issue, see here. Keeping an eye on this and will test again.

[trappitsch]

Well, pint seems to be fixed at this point, however, there is one more larger issue: python-vxi11. It depends on xdrlib (was part of the standard library), which was deprecated in 3.11 and removed in 3.13. A summary of the issues and a possible fix is discussed here: python-ivi/python-vxi11#49

Unfortunately, python-vxi11 seems to be not maintained anymore. This is a larger problem for ik, I'll put more thoughts into a new issue.