Introduce six to the codebase to deal with Python 2/Python 3 differences

[nicoddemus]

It would be nice to remove a good portion of our Python 2/Python 3 own compatibility layer. Ultimately we must see if the change is worthwhile (my initial feeling is that it is, but that is to be seen yet).

[RonnyPfannschmidt]

this also would remove a lot of the messy pylib dependencies

[nicoddemus]

Realized that we should discuss if six should be vendored or not, given that six also tests itself using pytest, similar to the discussion taking place in #2641. I guess the exact same arguments to vendor attrs or not apply to six.

[RonnyPfannschmidt]

important point -

[nicoddemus]

Fixed by #2650