[issue45975] Simplify some while-loops with walrus operator

[Irit Katriel]

Irit Katriel <iritkatriel at gmail.com> added the comment:

As I mentioned in the PR, merging this can make future backports to 3.6 and 3.7 more complicated.

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nosy: +iritkatriel

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<https://bugs.python.org/issue45975>
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