Issue #27431: Update default protocol version in shelve.Shelf() documentation

shelve.open() documentation was updated in f351fb7ea179.
diff --git a/Doc/library/shelve.rst b/Doc/library/shelve.rst
index 1ec158e..db66a63 100644
--- a/Doc/library/shelve.rst
+++ b/Doc/library/shelve.rst
@@ -108,7 +108,7 @@
    A subclass of :class:`collections.abc.MutableMapping` which stores pickled
    values in the *dict* object.
 
-   By default, version 0 pickles are used to serialize values.  The version of the
+   By default, version 3 pickles are used to serialize values.  The version of the
    pickle protocol can be specified with the *protocol* parameter. See the
    :mod:`pickle` documentation for a discussion of the pickle protocols.
 
diff --git a/Lib/test/test_shelve.py b/Lib/test/test_shelve.py
index bd51d86..b71af2b 100644
--- a/Lib/test/test_shelve.py
+++ b/Lib/test/test_shelve.py
@@ -162,6 +162,10 @@
         else:
             self.fail('Closed shelf should not find a key')
 
+    def test_default_protocol(self):
+        with shelve.Shelf({}) as s:
+            self.assertEqual(s._protocol, 3)
+
 from test import mapping_tests
 
 class TestShelveBase(mapping_tests.BasicTestMappingProtocol):