Use six in pyrevitlib

[sanzoghenzo]

Following #2811

we could use six to ensure compatibility with both python 2 and 3, instead of manually patching pyrevitlib's code.

As of now, the problem to introduce this is that site-packages, that is where six is placed, is not loaded by PyRevitLoaderApplication (as opposed to PyRevitRunner that loads both pyrevitlib and site-packages before running the user script).

We have two possible options:

1. add site-packages to the syspaths in PyRevitLoaderApplication
2. copy over the six module inside pyrevitlib. From the documentation:

   six consists of only one Python file, so it is painless to copy into a project

My thoughts: since #2647 is on the way, both options should be temporary, so option 1 would be better (no need to copy things around, and no need to change imports afterwards). But I don't know if this would introduce unwanted side effects.