Tracking issue for pinning sphinxcontrib-bibtex<2.0.0

[hugohadfield]

As mentioned in #381 a change in the upstream package sphinxcontrib-bibtex means we should pin our version to <2.0.0 . This is a tracking issue to remind us to keep an eye on future releases of sphinxcontrib-bibtex and how other read-the-docs based packages handle the transition, the relevant issue on sphinxcontrib-bibtex appears to be this one: mcmtroffaes/sphinxcontrib-bibtex#214