[issue2877] Backport UserString move from 3.0

[Brett Cannon]

Brett Cannon <brett at python.org> added the comment:

Is MutableString in the UserString module (and any other class) properly 
handled in 2.6 for transitioning to 3.0?

And there is currently no fixer for UserString (same for UserDict).

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<http://bugs.python.org/issue2877>
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