[issue19950] Document that unittest.TestCase.__init__ is called once per test

[Cheryl Sabella]

Change by Cheryl Sabella <chekat2 at gmail.com>:


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keywords: +easy -patch
stage:  -> needs patch
versions: +Python 3.7, Python 3.8 -Python 3.4

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<https://bugs.python.org/issue19950>
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