Removed tox.
While trying to get setup to contribute to pacman-mirrors, I kept running into issues with tox trying to run tests against Python 3.4. In a comment on the forum (https://forum.manjaro.org/t/getting-started-developing-pacman-mirrors/50674/16), @fhdk suggested we don't need to test against older Pythons, so I thought this might be a good first PR, to get the docs a little more clear.