[issue18797] Don't needlessly change refcounts of dummy objects for sets

[Roundup Robot]

Roundup Robot added the comment:

New changeset ac6dab1819c4 by Raymond Hettinger in branch 'default':
Issue 18797:  Remove unneeded refcount adjustments for dummy objects.
http://hg.python.org/cpython/rev/ac6dab1819c4

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nosy: +python-dev

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<http://bugs.python.org/issue18797>
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