Determine required Python versions from tox.ini and install if required

tox documentation suggests that the versions of Python that you want to test against must already be installed in the environment, otherwise you'll experience errors. With that in mind, is there a generally accepted practice of checking envlist in tox.ini and using that to then install the required versions. E.g. for:

envlist = py27, py36

check if these are installed and if not, install them.