checkdeps: check for sphinxcontrib-bibtex

I lacked this package, and got:
``` $ make -j2 COQ_USE_DUNE=1 refman-html
[...]
         env doc/sphinx_build (exit 2)
(cd _build/default/doc && /usr/bin/env COQLIB=.. sphinx-build -j4 -W -b html -d sphinx_build/doctrees sphinx sphinx_build/html)
Running Sphinx v2.1.2

Extension error:
Could not import extension sphinxcontrib.bibtex (exception: No module named 'sphinxcontrib.bibtex')
make: *** [refman-html] Error 1
```

0 files changed, 0 insertions, 0 deletions