gh-158093: Add concurrent test for set.pop() method

[CaQtiml]

Add a case for multiple threads calling set.pop() on the same set. The test checks that all original items are returned exactly once and the set ends up empty.

I only see a test for normal usage (Lib/test/test_set.py), but I do not see any case testing .pop concurrently, so I believe this case will add coverage for free-threaded Python.

Testing

• make patchcheck
• ./python.exe -B -X gil=0 -m test test_free_threading.test_set

• Issue: Add test coverage for concurrent set.pop() calls #158093

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