Some versions of our modules still support Python 2.6, but Sphinx 1.6+

has a dependency on the typing module, which does not. In order to

prevent development environments (such as from CI) from failing, and to

reduce the requirements for contributors, the docs-specific requirements

have been moved into doc-requirements.txt. These are completely

optional, and only needed when building the documentation.
These dependencies can be installed either by manually running

pip install -r doc-requirements.txt, or by passing --with-doc-deps

to setup.py develop.
The dependency versions have also been updated.
This is the same change that recently went into Djblets.

Tested with and without --with-doc-deps. Saw that the dependencies

did not install without it.