The default shelve pickle protocol should have been 3.

Doc/library/shelve.rst

@@ -23,7 +23,7 @@ lots of shared  sub-objects.  The keys are ordinary strings.
    database file is opened for reading and writing.  The optional *flag* parameter
    has the same interpretation as the *flag* parameter of :func:`dbm.open`.
 
-   By default, version 2 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.
 
    By default, mutations to persistent-dictionary mutable entries are not

Lib/shelve.py

@@ -84,7 +84,7 @@ class Shelf(collections.MutableMapping):
                  keyencoding="utf-8"):
         self.dict = dict
         if protocol is None:
-            protocol = 2
+            protocol = 3
         self._protocol = protocol
         self.writeback = writeback
         self.cache = {}

Misc/NEWS

@@ -157,6 +157,8 @@ Library
 
 - Issue #1581476: Always use the Tcl global namespace when calling into Tcl.
 
+- The shelve module now defaults to pickle protocol 3.
+
 - Fix a bug in the trace module where a bytes object from co_lnotab had its
   items being passed through ord().