Add support for conditional coverage rules during unit testing

[rmartin16]

Changes

• As was done for Briefcase in https://github.com/beeware/briefcase/pull/1262, Toga unit testing now supports conditional coverage for the Python version
• Closes #2640.

PR Checklist:

• [x] All new features have been tested
• [X] All new features have been documented
• [X] I have read the CONTRIBUTING.md file
• [X] I will abide by the code of conduct

[rmartin16]

First run today.... :(

RuntimeError: MapView web canvas didn't initialize

https://github.com/beeware/toga/actions/runs/9469019167/job/26086864938?pr=2642#step:6:642

[freakboy3742]

First run today.... :(

RuntimeError: MapView web canvas didn't initialize

Also an RTD fail because support.microsoft.com is down again... (grumble)