[issue2876] Write UserDict fixer for 2to3

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

UserDict.UserDict is gone.
UserDict.IterableUserDict class is now collections.UserDict.
UserDict.Mixin is now collections.MutableMapping.

The new classes comform to the new dict API, so they fixer needs to 
also make same method adaptatations as for dicts (i.e. d.keys() --> list
(d.keys() and d.has_key --> d.__contains__).

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Tracker <report at bugs.python.org>
<http://bugs.python.org/issue2876>
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