I updated the dev docs and merged the branch.

Looks OK. I assume the dev docs need to be updated with the new depedencies and setup steps. If this saves potentially large amounts of stuff in the misc/.tox directory that hg status will not show, then this also needs to be clearly stated, along with explaining the necessary clean-up steps.

I made a pull request for this at 
https://bitbucket.org/jendrikseipp/downward/pull-requests/138 .

Sounds good.

That's a valid concern. I think testing multiple Python 3 versions is useful. Also, switching to tox allows us to use the same version of flake8 and pytest everywhere and developers don't need to install these packages manually and globally. I'll give this a try and will abandon it if it turns out too complicated.

Is it worth bothering with extra work to improve the support for multiple Python versions at the "meta level" at this point?

With all the high-profile projects that drop support for Python 2 on January 1, I expect that it will soon become a lot more viable for us to drop Python 2 support too. Would this work still be useful then, e.g. to test multiple versions of Python 3?

Currently, we test the translator and the driver only under Python 2. I plan to use tox for testing multiple Python versions.