[issue42138] Forward port Doc/whatsnew/3.9.rst to master

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

Terry did the same thing in https://bugs.python.org/issue42139.

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resolution:  -> duplicate
stage: patch review -> resolved
status: open -> closed

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42138>
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