siglent power backend incompatible with Python 3.13

[Bastian-Krause]

The siglent power backend uses the vxi11 module. It hasn't seen any activity in the last 7 years. It still uses the deprecated xdrlib module from the Python standard library. xdrlib will be removed in Python 3.13 which is expected next week.

Upstream issue: https://github.com/python-ivi/python-vxi11/issues/49

If nobody steps in, we will deprecate the siglent power backend and drop it in the foreseeable future.

Cc @esben (original contributor of the siglent power backend)