Upgrade Python version

[Karl Dubost]

Heroku is telling us

-----> Python app detected

 !     The latest version of Python 3.6 is python-3.6.6 (you are using python-3.6.5, which is unsupported).

 !     We recommend upgrading by specifying the latest version (python-3.6.6).

       Learn More: https://devcenter.heroku.com/articles/python-runtimes