Use 'dev' as the version for development documentation builds
This PR modifies the sphinx configuration to just use dev
as the version number when building outside of tags. This is needed since the tags aren't always synchronised to the development branch (since we use a release/X.Y.z
branch for bug-fix releases). E.g. the current dev docs say 2.0.1
because the 2.0.2-2.0.4 tags aren't on master
.