I'm doing this because it makes sense to separate which requirements are
needed to develop the package and which are purely needed for static code
analysis. That said, the trigger for this commit was that ReadTheDocs
automatically tries to install everything that's in requirements.txt and,
since `black` isn't available on Python 2.7, it was failing. See
https://readthedocs.org/projects/mongoengine-odm/builds/9371765/.
Refs #2105
