Chris Jerdonek added the comment:
I think we generally upgrade it exactly when there is a new Sphinx feature we want for our docs :)
:) Would the appropriate way to handle it be then to create an issue to "upgrade Sphinx when XXX issue is resolved" and link to the corresponding Sphinx issue?
I also see value in Python being able to track and possibly assist Sphinx in fixing an issue that we want.
Incidentally, issue 15455 may fall into the same category now that I know more.