[issue37669] Make mock_open return per-file content

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan <tir.karthi at gmail.com>:


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nosy: +cjw296, mariocj89, michael.foord, xtreak
versions:  -Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37669>
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