[issue17883] Fix buildbot testing of Tkinter

[Roundup Robot]

Roundup Robot added the comment:

New changeset 358496e67a89 by Zachary Ware in branch '2.7':
Issue #17883: Backport test.test_support._is_gui_available()
http://hg.python.org/cpython/rev/358496e67a89

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<http://bugs.python.org/issue17883>
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