[issue42396] Add a whatsnew entry about async contextlib.nullcontext

[Andrew Svetlov]

Change by Andrew Svetlov <andrew.svetlov at gmail.com>:


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions: +Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42396>
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