Update to requiring a later version of Sphinx
Update to requiring a later version of Sphinx

We've been on 1.8.5 for a while, in part due to not being ready to switch to
Python 3 full time.  Now that we want to start relying on Python 3, update the
version of Sphinx we use.

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Signed-off-by: Lydia Duncan <f955cf5f42dff0db8f444c4740a67a0cf0b4805d@users.noreply.github.com>

chapel-lang/sphinxcontrib-chapeldomain