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, PySOA will support only Python 3.7 and above and will use Python Typing syntax instead of Python Typing comments, removing try/except around the existing AsyncIO support, eliminate the use of the six library, and remove backports (enum, typing, etc.) from the list of dependencies. Some or all Attrs classes may possibly switched to dataclasses, as well.