[docs] [issue38062] Clarify that atexit.unregister matches by equality, not identity

[Mark Dickinson]

Change by Mark Dickinson <dickinsm at gmail.com>:


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assignee:  -> docs at python
components: +Documentation
nosy: +docs at python
versions: +Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38062>
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