UserString.MutableString has been removed in Python 3.0.
Works on issue #2877. Thanks Quentin Gallet-Gilles for the patch.
diff --git a/Doc/library/userdict.rst b/Doc/library/userdict.rst
index 7d36401..b0ef645 100644
--- a/Doc/library/userdict.rst
+++ b/Doc/library/userdict.rst
@@ -178,6 +178,9 @@
mutable object as dictionary key, which would be otherwise very error prone and
hard to track down.
+ .. deprecated:: 2.6
+ The :class:`MutableString` class has been removed in Python 3.0.
+
In addition to supporting the methods and operations of string and Unicode
objects (see section :ref:`string-methods`), :class:`UserString` instances
provide the following attribute: