I'm hitting the same problem that pytest hit a while back:
https://ci.appveyor.com/project/CherryPy/cheroot/builds/27622282/job/6etbt4hn5hv08x3y#L132

So basically tox does pip install 'pip >= 9' 'tox-run-command >= 0.4' 'tox >= 3.13.2' in .tox venv but Pip that is invoked is Pip 8.
So this pulls in more-itertools via as a dependency choosing v7.0.0 which is incompatible with Python 2.

I think, it's reasonable to upgrade Pip as a pre-requisite step.