[issue18772] Fix gdb plugin for new sets dummy object

[Roundup Robot]

Roundup Robot added the comment:

New changeset be29efa60b68 by Raymond Hettinger in branch 'default':
Issue 18772:  Restore set dummy object back to unicode and restore the identity checks in lookkey().
http://hg.python.org/cpython/rev/be29efa60b68

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nosy: +python-dev

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<http://bugs.python.org/issue18772>
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