gh-117865: Defer several imports in inspect.py

[AlexWaygood]

Following 153b3f7, it is suddenly much more trivial to make the import of ast lazy in inspect.py. Once that's done, it also becomes worthwhile to make other imports lazy as well

• Issue: Inspect module is prohibitively expensive to import #117865

[AlexWaygood]

I'm about to board a flight, but I think this cuts nearly 50% off the import time for inspect

[AlexWaygood]

(But this is possibly a redundant optimisation if it turns out that inspect needs to depend on ast more heavily due to PEP 649 anyway? Not sure)

[AlexWaygood]

I'll keep this on the back burner until it's decided where the new PEP-649 functions will go, and which modules they'll depend on

[JelleZijlstra]

(This showed up in my list of review requests, so I want to clear it out even if it's a draft.)

I think we can apply the changes that don't affect ast. My current implementation of the PEP 649 functionality relies very heavily on ast, so it's likely we won't be able to defer that import.

[AlexWaygood]

I think we can apply the changes that don't affect ast.

Right -- but ast itself imports a bunch of stuff, so I'm not sure how much deferring the other imports helps if we can't defer the ast import. It might just make the code uglier for no apparent gain. I'll play around a bit when I get a chance; maybe now we've deferred the re import in ast things might have changed...