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

[Tom Gringauz]

Change by Tom Gringauz <tomgrin10 at gmail.com>:


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keywords: +patch
pull_requests: +22250
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23357

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