This simplifies getting going with a Review Board source tree. Before,

Review Board would require a few steps to get set up, which included

installing running setup.py develop (and having everything in place to

compile Pillow and other modules), installing the packages in

dev-requirements.txt, and installing the required node modules. That was

a lot of steps, and caused problems for those getting started

(particularly as some complex dependencies are no longer shipped as

eggs).
Now, setup.py develop takes care of installing dependencies as wheels,

installing development packages, and installing modules from npm. It's a

far simpler process than we've had in the past.
Like in the accompanying Djblets change, the package dependencies now

live in a reviewboard.dependencies module. This will help simplify

things for us in some ways (such as with RBCommons packaging and

extension packaging).
The prepare-dev.py script has been updated to no longer install

modules itself, and to allow develop to do more work.

Used a new virtualenv consisting of just the Djblets source tree and

dependencies. Used a clean copy of the Review Board source tree. Ran

./setup.py develop and saw it install wheels and node modules. Ran

unit tests and they passed.
Created a new environment and installed Djblets into it. Then I set up

the Review Board tree using ./contrib/internal/prepare-dev.py and

repeated tests. Everything worked.