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

[Ned Deily]

Ned Deily <nad at python.org> added the comment:

Julien (@mdk) is the doc builds expert.

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assignee: docs at python -> mdk

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