Neither pip nor virtualenv install python (though pip tries). They use whatever you specify.
You could write a README that mentions required Python version or provide a fabric script that can deploy to localhost and specify the version there. For example, instructions to install virtualenv, pip, distribute.
For those people who don’t read instructions there could be a CI system (jenkins, buildbot) that can run unit-tests using supported python versions (before/after commit).
To manage multiple python installation you could use something like pythonz.