gh-117398: datetime: Make use of C-API capsules for sub-interpreters

[neonene]

Allow sub-interpreters to access their C-API by calling PyCapsule_Import function every time. Performance could be recovered by client's local caching. The _zoneinfo extension could also improve a few C-API accesses, if necessary.

UPDATE: Tuned for a sub-interpreter: #117413 (comment)

cc @erlend-aasland@ericsnowcurrently

• Issue: Isolate the _datetime extension module #117398

[ericsnowcurrently]

I'm especially interested to know what @pganssle thinks.

[neonene]

#117498 is a demo version of _datetime isolated with this way, which I think does not conflict with long-term approaches.

cc @pganssle

[neonene]

I have made this the same as #118357 except that this PR caches capi-pointers in the global C array rather than PyInterpreterState. One subinterpreter can use the global variable for now, so PyCapsule_Import will be called when multiple subinterpreters use the _datetime module at the same time.