Add full tests + Ruff autofixes

[Avasam]

Added full test suite + Autofixable code modernizations from Ruff
No manual code change.

---

Account for lack of primary monitor in tests

[Avasam]

ohh, interesting, Linux test failing I think because no virtual monitor is being returned as the primary so _XgetMonitorData fails the monitor.primary == 1 and returns None. Not immediatly sure if that's a bad test expectation, or if _XgetMonitorData should have a fallback.
@Kalmat your know your API and intent better than I do !

[Kalmat]

It should definitely have a fallback in case there is no primary monitor. Not the typical case, but totally acceptable in Linux (not in Windows nor macOS). I will work on it after we finish.

Thanks!!