[docs] [issue26638] Avoid warnings about missing CLI options when building documentation

[Zachary Ware]

Zachary Ware added the comment:

Terry: for now, you can downgrade Sphinx to 1.3.6 as it is known to build with no warnings (that's the version the Docs buildbot currently uses).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue26638>
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