bpo-29845: Mark tests that use _testcapi as CPython-only (#711) (#725)

(cherry picked from commit 24c738a9e91b8f46da6166663d8ce7ec18cec784)
diff --git a/Lib/test/test_atexit.py b/Lib/test/test_atexit.py
index 172bd25..c761076 100644
--- a/Lib/test/test_atexit.py
+++ b/Lib/test/test_atexit.py
@@ -143,6 +143,7 @@
         self.assertEqual(l, [5])
 
 
+@support.cpython_only
 class SubinterpreterTest(unittest.TestCase):
 
     def test_callbacks_leak(self):