gh-125859: Fix crash when gc.get_objects is called during GC (#125882)

This fixes a crash when `gc.get_objects()` or `gc.get_referrers()` is
called during a GC in the free threading build.

Switch to `_PyObjectStack` to avoid corrupting the `struct worklist`
linked list maintained by the GC. Also, don't return objects that are frozen
(`gc.freeze()`) or in the process of being collected to more closely match
the behavior of the default build.

Lib/test/test_gc.py

@@ -1065,6 +1065,29 @@ class GCTests(unittest.TestCase):
         self.assertEqual(len(gc.get_referents(untracked_capsule)), 0)
         gc.get_referents(tracked_capsule)
 
+    @cpython_only
+    def test_get_objects_during_gc(self):
+        # gh-125859: Calling gc.get_objects() or gc.get_referrers() during a
+        # collection should not crash.
+        test = self
+        collected = False
+
+        class GetObjectsOnDel:
+            def __del__(self):
+                nonlocal collected
+                collected = True
+                objs = gc.get_objects()
+                # NB: can't use "in" here because some objects override __eq__
+                for obj in objs:
+                    test.assertTrue(obj is not self)
+                test.assertEqual(gc.get_referrers(self), [])
+
+        obj = GetObjectsOnDel()
+        obj.cycle = obj
+        del obj
+
+        gc.collect()
+        self.assertTrue(collected)
 
 
 class IncrementalGCTests(unittest.TestCase):