[issue1367711] Remove usage of UserDict from os.py

[Raymond Hettinger]

Raymond Hettinger added the comment:

> it's better to derived from dict for things like
> this than to derive from UserDict or UserDictMixin

That's true for UserDict but not DictMixin.  IIRC, 
the latter will continue to exist in Py3.0 and it
has a purpose that is much different than UserDict
or dict, namely to build-out objects with a mapping
interface but are *not* dicts under the hood.

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