Based on this comment exchange https://github.com/mozilla/mozdownload/pull/185#discussion_r8155442 I assumed that we can assume that virtualenv is installed on the tester machine.

See also davehunt's comment at the end.

I am not quite sure how to download virtualenv, not install it, but create a virtual environment, to be honest. What is the accepted approach here? Researching the topic came up with some solutions, but I'd like to know the mozilla way.