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

(cherry picked from commit 24c738a9e91b8f46da6166663d8ce7ec18cec784)
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index 1508f20..2497e47 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -906,6 +906,7 @@
             self.assertEqual(swapped & mask, mask)
             self.assertRaises(OverflowError, func, 1<<34)
 
+    @support.cpython_only
     def testNtoHErrors(self):
         good_values = [ 1, 2, 3, 1, 2, 3 ]
         bad_values = [ -1, -2, -3, -1, -2, -3 ]