gh-129967: Fix race condition in repr(set) (gh-129978)

The call to `PySequence_List()` could temporarily unlock and relock the
set, allowing the items to be cleared and return the incorrect
notation `{}` for a empty set (it should be `set()`).

Co-authored-by: T. Wouters <thomas@python.org>

Lib/test/test_free_threading/test_set.py

@@ -0,0 +1,41 @@
+import unittest
+
+from threading import Thread, Barrier
+from unittest import TestCase
+
+from test.support import threading_helper
+
+
+@threading_helper.requires_working_threading()
+class TestSet(TestCase):
+    def test_repr_clear(self):
+        """Test repr() of a set while another thread is calling clear()"""
+        NUM_ITERS = 10
+        NUM_REPR_THREADS = 10
+        barrier = Barrier(NUM_REPR_THREADS + 1)
+        s = {1, 2, 3, 4, 5, 6, 7, 8}
+
+        def clear_set():
+            barrier.wait()
+            s.clear()
+
+        def repr_set():
+            barrier.wait()
+            set_reprs.append(repr(s))
+
+        for _ in range(NUM_ITERS):
+            set_reprs = []
+            threads = [Thread(target=clear_set)]
+            for _ in range(NUM_REPR_THREADS):
+                threads.append(Thread(target=repr_set))
+            for t in threads:
+                t.start()
+            for t in threads:
+                t.join()
+
+            for set_repr in set_reprs:
+                self.assertIn(set_repr, ("set()", "{1, 2, 3, 4, 5, 6, 7, 8}"))
+
+
+if __name__ == "__main__":
+    unittest.main()