gh-139716: StackRef tests are added

[efimov-mikhail]

New test file Lib/test/test_stackrefs.py introduced.
Some auxillary functions are added to _testinternalcapi.
Those functions are examples of correct StackRef scenarios.
We check that all refcounts remain correct.
And that results are the same for equivalent scenarios.

• Issue: Add tests on basic PyStackRef functions #139716

[efimov-mikhail]

CC @markshannon @Fidget-Spinner @mpage

[markshannon]

Looks good. I've one concern about code organization, but that's all.

[markshannon]

I worry that stackref_to_tuple is likely to get out of sync when the stackref implementation changes. Can you move it to Python/stackrefs.c, rename it to _PyStackRef_AsTuple and a declaration to pycore_stackref.h

The other functions below are fine here.

[efimov-mikhail]

I've done this change. But I can see no much value in moving this function to Python/stackrefs.c, so I create 3 different implementations of this little function at pycore_stackref.h.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[efimov-mikhail]

I have made the requested changes; please review again.

[bedevere-app]

Thanks for making the requested changes!

@markshannon: please review the changes made to this pull request.

[sergey-miryanov]

Thanks!