Install sphinx when publishing docs (#1404)
Install sphinx when publishing docs (#1404)

A previous build performance optimisation (#1379, commit 889a42a) sped
things up by installing sphinx only when checking that docs build.

However, this meant that sphinx wasn't installed when we come to
actually building the docs (which doesn't happen on pre-merge builds).

This addition should mean we can publish docs again.
[no release notes]
EvilMcJerkface/atlasdb,EvilMcJerkface/atlasdb,palantir/atlasdb,EvilMcJerkface/atlasdb,palantir/atlasdb,palantir/atlasdb