Predict the Python version based on $VIRTUAL_ENV Try to make prediction of the Python's site-package version used by virtualenv. If predicted path doesn't exist, continue with previous behaviour. This can be useful while working with different Python versions in virtual environments other than system-used (previously provided via `sys.version_info`). Note that this *only* updates $PYTHONPATH, so 3rd-libraries installed in virtualenv will be avialable. However the Python interpreter contiunues to be other than the bundled in the virtualenv (see `init_virtualenv()` docstring).