gh-123619: Add an unstable C API function for enabling deferred reference counting

[ZeroIntensity]

cc @Fidget-Spinner

• Issue: Instance method performance issue with free threading (3.13rc1) #123619

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📚 Documentation preview 📚: https://cpython-previews--123635.org.readthedocs.build/en/123635/c-api/object.html#c.PyUnstable_Object_SetDeferredRefcount

[cpython-cla-bot]

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[ZeroIntensity]

Looks like my git email got screwed up... I'll fix that later.

[Fidget-Spinner]

@colesbury

[colesbury]

I think we want this or something similar, but it needs to go through the C API working group first: https://github.com/capi-workgroup/decisions/

[ZeroIntensity]

Sounds good. Though, there might be some technical issues here that I'm not thinking of. I proposed something similar using immortal objects rather than DRC, but it seems that we just can't ensure enough support for it to be worthwhile.

[colesbury]

As @encukou wrote in capi-workgroup/decisions#42, this needs a test (in _testcapimodule.c). I think the primary thing we can test is that the preconditions are enforced and that the return codes are correct.

[ZeroIntensity]

I'll get to updating the rest of this sometime later today.

[ZeroIntensity]

I've added a test. At this point, is the _Py_IsOwnedByCurrentThread check still needed?

[colesbury]

LGTM.

@encukou, would you please take another look at this?

[encukou]

This looks good, thank you!