software/slapos-testing: use python3.13 in software-py3next.cfg

This MR tracks the minimum work to have software/slapos-testing/software-py3next.cfg compiling with python3.13.

Depends on:

• https://lab.nexedi.com/nexedi/pygolang/-/merge_requests/39
• https://lab.nexedi.com/nexedi/wendelin.core/-/merge_requests/43