Documentation does not build with latest version of sphinx

Today sphinx 1.7.2 was released, and I needed to add this commit to pin sphinx to 1.7.1 and pass CI again:

commit f9edceed17039df045677f7103c40f7dbadf2a5d
Author: Tristan Van Berkom <tristan.vanberkom@codethink.co.uk>
Date:   Wed Mar 21 23:23:28 2018 +0900

    .gitlab-ci.yml: Pin the sphinx version to 1.7.1
    
    The new version today 1.7.2 causes the following build error:
    
        Warning, treated as error:
        /builds/BuildStream/buildstream/dist/buildstream/doc/source/buildstream.sandbox.rst:document isn't included in any toctree