Move dev reqs from tox.ini to dev-requirements.txt

Then folks who don't use tox can do `pip install -r
dev-requirements.txt`

Since this is pip, which created the idea of requirements files, it
seems nice for the project to have one to showcase the idea.

Also cleaned up the tox.ini and made it more compact (fits now on my
laptop screen) and consistent.
Merge pull request #2220 from msabramo/dev-requirements.txt

Move dev reqs from tox.ini to dev-requirements.txt