Use sys.monitoring (PEP-669) for reduced overhead in the explain phase on Python 3.12+

[Zac-HD]

As part of the explain phase, codenamed Scrutineer, we collect and analyze coverage information. This currently uses Python's sys.settrace() functionality, which provides hooks on which coverage analysis (or debuggers) can be implemented - at the cost of some performance overhead. From Python 3.12, PEP-669 offers a lower-overhead alternative in the sys.monitoring module.

Reimplementing the data-collection logic of Scrutineer using this new API offers us a basically free performance boost - so while the implementation might be tricky and it's not a huge priority while so few people have dropped older Python versions, I'm keen!