Adds py37 to the env list and switches pep8 python to
python3, including necessary changes to make pylint
and pep8 checks happy in python3:
* Unpins pylint, as 1.4.5 explodes (exception) when trying
to check our code.
* Unpins flake8-import-order from an old version.
* Stops making use of the builtin cmp function, and replaces
it with the recommended (a > b) - (a < b).
* Disables one py3 specific warning, since it's still necessary
for python2 (useless-object-inheritance)
* Removes a non existing argument in netprobe parser (version).
* Inserts an additional blank line between __future__ imports
and stdlib imports.