UserString.MutableString has been removed in Python 3.0.

Works on issue #2877. Thanks Quentin Gallet-Gilles for the patch.
diff --git a/Misc/NEWS b/Misc/NEWS
index e5f251a..b3202b8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -63,6 +63,9 @@
 Library
 -------
 
+Issue #2877 - The UserString.MutableString class has been removed in
+			  Python 3.0.
+
 - Do not close external file objects passed to tarfile.open(mode='w:bz2')
   when the TarFile is closed.