setup.py develop should install all development requirements

[raphaelcastaneda]

It shouldn't be necessary to run: pip install -r requirements_dev.txt

Instead, setup.py develop should include these requirements as well

[xelaadryth]

Development mode for setuptools seems more for running the code without having to rebuild it, rather than actually setting up developer dependencies. I think there will always be a separate step for installing developer dependencies regardless, but we could still simplify it by using extras_require and adding an entry for a dev install in setup.py and instead running:

pip install -e .[dev]

This would install the setup.py install as well as dev dependencies in one command.

However, this also breaks tox dependencies which can take a requirements file, so probably better not to change it.