gh-126312: Don't traverse frozen objects on the free-threaded build (#126338)

Also, _PyGC_Freeze() no longer freezes unreachable objects.

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Lib/test/test_gc.py

@@ -1082,6 +1082,44 @@ class GCTests(unittest.TestCase):
         gc.collect()
         self.assertTrue(collected)
 
+    def test_traverse_frozen_objects(self):
+        # See GH-126312: Objects that were not frozen could traverse over
+        # a frozen object on the free-threaded build, which would cause
+        # a negative reference count.
+        x = [1, 2, 3]
+        gc.freeze()
+        y = [x]
+        y.append(y)
+        del y
+        gc.collect()
+        gc.unfreeze()
+
+    def test_deferred_refcount_frozen(self):
+        # Also from GH-126312: objects that use deferred reference counting
+        # weren't ignored if they were frozen. Unfortunately, it's pretty
+        # difficult to come up with a case that triggers this.
+        #
+        # Calling gc.collect() while the garbage collector is frozen doesn't
+        # trigger this normally, but it *does* if it's inside unittest for whatever
+        # reason. We can't call unittest from inside a test, so it has to be
+        # in a subprocess.
+        source = textwrap.dedent("""
+        import gc
+        import unittest
+
+
+        class Test(unittest.TestCase):
+            def test_something(self):
+                gc.freeze()
+                gc.collect()
+                gc.unfreeze()
+
+
+        if __name__ == "__main__":
+            unittest.main()
+        """)
+        assert_python_ok("-c", source)
+
 
 class IncrementalGCTests(unittest.TestCase):