Issue #20532: Tests which use _testcapi now are marked as CPython only.
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index acf6d36..c1523db 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -610,6 +610,7 @@
         ret, out, err = assert_python_ok(*args)
         self.assertIn(b"free PyDictObjects", err)
 
+@test.support.cpython_only
 class SizeofTest(unittest.TestCase):
 
     def setUp(self):