While this makes things a little more complicated and means we'll need to use
`-r` to recreate tox environments, it has several advantages:
- Full support from requires.io (including PRs)
- Workaround for https://bitbucket.org/hpk42/tox/issues/332/ so we can update
virtualenv/pip
Having a Python script was a good idea back when we could do almost the
same steps on every CI configuration.
This turned out to grow into a complicated script, so it's easier to
split off things for Linux/OS X into a small shell script (and keep
Python for Windows, as I really don't want to use .bat/.ps).
