[python3] RHEL 8 will include python 3.6

Now that the RHEL 8 beta is released, there is more official information about what is included. In particular, the question we've been waiting to have answered is "which version of python 3 will be the default?" The answer is 3.6, which means it should be safe to drop 3.5 testing for Stein. https://developers.redhat.com/blog/2018/11/14/python-in-rhel-8/ -- Doug