[issue18944] Minor mistake in test_set.py

[Roundup Robot]

Roundup Robot added the comment:

New changeset de6047ea33e6 by Terry Jan Reedy in branch '2.7':
Issue #18944: backport typo fix
http://hg.python.org/cpython/rev/de6047ea33e6

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<http://bugs.python.org/issue18944>
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