Remove all Python 2.7-3.6 support

[nickwilliams-eventbrite]

This is the tracking issue for removing all support for Python 2.7 and 3.5-3.6. Once closed, Conformity will support only Python 3.7 and above and will use Python Typing syntax instead of Python Typing comments, eliminate the use of the six library, and remove backports (enum, typing, etc.) from the list of dependencies