[issue18357] add tests for dictview set difference operations

[Fraser Tweedale]

Fraser Tweedale added the comment:

and here's a patch for python3 (the earlier patch is for the 2.7 branch).

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Added file: http://bugs.python.org/file30773/dictview-set-sub-test-python3.patch

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<http://bugs.python.org/issue18357>
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