Upgrade Sphinx for Stem's site

In #10411 (closed) we made our tutorial examples downloadable via a link that accompanies the snippets. This is neat! But it relies on a Sphinx attribute added in version 1.3. This presently isn't in the Debian backports...

https://packages.debian.org/source/sphinx

I'm disabling this feature until it is so we can continue to build the site. Once it's in backports the host Stem's site is built on should upgrade, and we can roll forward.