[issue45975] Simplify some while-loops with walrus operator

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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pull_requests: +29290
pull_request: https://github.com/python/cpython/pull/31107

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45975>
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