[issue29620] unittest.TestCase.assertWarns raises RuntimeEror if sys.modules changes size

[Serhiy Storchaka]

Change by Serhiy Storchaka <storchaka+cpython at gmail.com>:


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nosy: +serhiy.storchaka
versions: +Python 2.7, Python 3.8 -Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue29620>
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