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 fcca676..7277547 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -489,6 +489,7 @@
         p.wait()
         self.assertIn(executable, ["''", repr(sys.executable)])
 
+@test.test_support.cpython_only
 class SizeofTest(unittest.TestCase):
 
     def setUp(self):