Proposal: Adopt the six library, or drop Python 2 support

[tarkatronic]

I've seen that there are a number of utilities in this library for dealing with the disparities between Python 2 and 3. This is a point where the six library can be immensely helpful, in replacing this functionality as well as providing much more.

Alternatively, given that Python 2.7 will no longer be supported after the end of 2019, it might be a good time to simply start ripping out that support, and release a new major revision which is Python 3 only.

[prschmid]

Since Python 2.7 is still supported until the end of 2019, let's keep it supported at least until then. As such, using the six library is the way we'd want to go to make sure we don't leave anyone high and dry.

[tarkatronic]

@prschmid I saw that you dropped Python 2 from the docs and testing grid. With that, I would love to put in a bit of additional work to rip out any Python 2 shims which exist!

[prschmid]

Thanks for your willingness to put in that effort!

Yes, I indeed dropped the official support for it since 2.7 EOL was in January. That being said, I'd like to keep unofficial support for it for a bit longer since I have no idea the environment that people are running things in. Having the (unofficial) python 2.7 support isn't a great overhead at this moment, so, let's keep it around a bit longer.