This removes all of the python code for handling python 2.x vs 3.x, now
only 3.6+ is supported. This also drops all uses of the six module, as
its no longer needed.

Python 2.x and <= 3.5 are all EOL, it doesn't make sense to continue to
support version of python that are at the end of their lives and are
being removed from operating systems.

Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <mesa/piglit!223>