Explicitly drop Python2 support

[rossbar]

I just wanted to start a discussion about whether there is a need/desire for explicit Python2 support. There is no building/testing against Python2 currently, and I doubt things would work. IMO no should be using Python2 at this point, but if there are external users to whom commitments have been made for legacy support then we'll need to put in a lot of effort here.

Assuming we don't need to support Python2 any longer, there is a lot of cleanup that should be done. This is mostly relevant for linters etc. (see #130), which will undoubtedly flag all the Python2 specific patterns for removal.

[msschwartz21]

@vanvalen can confirm, but I don't think we have any commitments made to maintain Python2 support. I am fine with dropping it.