bpo-34204: Use pickle.DEFAULT_PROTOCOL in shelve (GH-19639)



Use pickle.DEFAULT_PROTOCOL (currently 5) in shelve instead of a
hardcoded 3.
diff --git a/Lib/test/test_shelve.py b/Lib/test/test_shelve.py
index ac25eee..cfdd67c 100644
--- a/Lib/test/test_shelve.py
+++ b/Lib/test/test_shelve.py
@@ -1,6 +1,8 @@
 import unittest
 import shelve
 import glob
+import pickle
+
 from test import support
 from test.support import os_helper
 from collections.abc import MutableMapping
@@ -160,7 +162,7 @@ def test_with(self):
 
     def test_default_protocol(self):
         with shelve.Shelf({}) as s:
-            self.assertEqual(s._protocol, 3)
+            self.assertEqual(s._protocol, pickle.DEFAULT_PROTOCOL)
 
 from test import mapping_tests