Tox installs the wrong version of pip to it's virtual env
This was apparently the result of the "virtualenvs" python package containing a pre-selected group of python packages that it refers to, one of which was the latest and greatest pip.
I don't know if this is the preferred way of doing this, but I found success by running
pip uninstall virtualenv
And then reinstalling with the version that worked
pip install virtualenv==15.0.1
With the "correct" version of virtualenv in place, I was able to run my tox command
source .tox/py27/bin/activate
and see the desired version of pip
pip --version
pip 8.0.3
A workaround for this is here: https://github.com/pypa/pip/issues/3666
Although to make it work I had to write "pip install pip==8.1.1" in my script. So to recap:
Add a pip.sh script to your project:
#!/bin/bash
pip install pip==8.1.1
pip install "$@"
Add to your tox.ini:
install_command = {toxinidir}/pip.sh {opts} {packages}