[docs] [issue35605] backported patch requires new sphinx, minimum sphinx version was not bumped

[Anthony Sottile]

Change by Anthony Sottile <asottile at umich.edu>:


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keywords: +patch, patch, patch
pull_requests: +10704, 10705, 10706
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35605>
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