CI: test Python 3.13 only (ravendb client requires 3.10+)

[poissoncorp]

Follow-up to #33. The post-merge CI on v7.2 failed on the ubuntu 3.9 job.

Root cause

Not our code — the upstream ravendb client uses a PEP 604 X | Y union as a runtime value (ravendb/documents/operations/ongoing_tasks.py:422), which only works on Python 3.10+. On 3.9 it raises TypeError: unsupported operand type(s) for |: 'type' and 'type' at import, so the whole suite fails to load. Python 3.9 cannot work regardless of CI.

Change

Test Python 3.13 only, keeping .NET 8 + 10 and both OSes:

• ubuntu-latest, 3.13, .NET 10 (forced)
• ubuntu-latest, 3.13, .NET 8 (default)
• windows-latest, 3.13, .NET 10 (forced)

(The two 3.13 jobs already passed in the failing run; .NET 8 moves onto a 3.13 job, which imports fine.)

Note (not changed here)

setup.py still declares python_requires=">=3.9", which is effectively inaccurate given the client needs 3.10+. Worth bumping to >=3.10 in a future release.