[issue39375] Document os.environ[x] = y and os.putenv() as thread unsafe

[Joannah Nanjekye]

Change by Joannah Nanjekye <nanjekyejoannah at gmail.com>:


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nosy: +nanjekyejoannah

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39375>
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