gh-138045: Fix importing numpy in legacy subinterpreters by skipping switch to main interpreter when not needed

[jan-hasse]

(singlephase == NULL) check in update_global_state_for_extension could be removed because it's only called with pointers to stack objects.

I've tried creating a minimal working reproducer instead of importing numpy with the example from #138045 (comment) but didn't manage to integrate that into Python's test suite. Maybe someone else has more knowledge on how to create a simple test that reproduce the numpy situation.

The fix can be backported down to 3.13.

• Issue: Python 3.13 breaks circular imports through single-phase extension modules using non-isolated subinterpreters #138045

[python-cla-bot]

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[ZeroIntensity]

Please add a news entry, and ideally a test case. This seems difficult to test, though. You could look into adding something to _testsinglephase.

[jan-hasse]

Added a news entry. I was not able to implement a test, I already looked at _testsinglephase but wasn't able to setup something similar to numpy :(

[ZeroIntensity]

Thanks for the contribution. See my comment -- this was already fixed by #144602.