[issue19704] Update test.test_threaded_import to PEP 451

[Roundup Robot]

New submission from Roundup Robot:

New changeset dbb9c23e1887 by Brett Cannon in branch 'default':
Issue #19704: Port test.test_threaded_import to PEP 451
http://hg.python.org/cpython/rev/dbb9c23e1887

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nosy: +python-dev

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19704>
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