Issue #26013: Added compatibility with broken protocol 2 pickles created

in old Python 3 versions (3.4.3 and lower).

Lib/_compat_pickle.py

@@ -177,6 +177,13 @@ IMPORT_MAPPING.update({
     'DocXMLRPCServer': 'xmlrpc.server',
     'SimpleHTTPServer': 'http.server',
     'CGIHTTPServer': 'http.server',
+    # For compatibility with broken pickles saved in old Python 3 versions
+    'UserDict': 'collections',
+    'UserList': 'collections',
+    'UserString': 'collections',
+    'whichdb': 'dbm',
+    'StringIO':  'io',
+    'cStringIO': 'io',
 })
 
 REVERSE_IMPORT_MAPPING.update({

Misc/NEWS

@@ -131,6 +131,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #26013: Added compatibility with broken protocol 2 pickles created
+  in old Python 3 versions (3.4.3 and lower).
+
 - Issue #26129: Deprecated accepting non-integers in grp.getgrgid().
 
 - Issue #25850: Use cross-compilation by default for 64-bit Windows.