[docs] [issue28796] FIX warnings in documentation build

[Roundup Robot]

Roundup Robot added the comment:

New changeset f16f0500b49b by Berker Peksag in branch 'default':
Issue #28796: Silence Sphinx warnings
https://hg.python.org/cpython/rev/f16f0500b49b

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nosy: +python-dev

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28796>
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