Regarding Python 3 currently our CI allows only to run tests with Python 3.4, that is less than a year from end of life (2019/03). In production we're quickly moving towards Debian Stretch that ships 3.5 and is most likely that newer developed software might not be backward compatible with Python 3.4. I've already a repo in which this is true.
Given that is possible to have multiple Python versions installed on a host, I think that our CI should have multiple Python versions installed allowing tox to run tests on multiple Python versions as defined in the tox.ini file. I'd say 2.7, 3.4, 3.5 and 3.6 should all be available.