Python 3.6

[drvinceknight]

Python 3.6 was released over the Christmas break (https://docs.python.org/3/whatsnew/3.6.html).

We have discussed "supporting the latest two versions of py3" as a general policy.

On a branch (py3.6) I've pushed the simple change to our travis build config: removing py 3.4 and adding py 3.6 (and a minor doctest compatibility fix).

One of the motivations for dropping 3.4 is that we can tackle #347 by using type annotations (which are available in both 3.5 and 3.6 but not 3.4).

Assuming that's all fine, I'll open a PR with that branch but one question: what do we do with releases (x.y.z)? I would be happy to simply bump y.

FYI, here's the travis build with 3.5 and 3.6: https://travis-ci.org/Axelrod-Python/Axelrod/builds/187938394

CC: @marcharper @meatballs

:) 👍

[marcharper]

Sounds good to me. We should also look into using enums, introduced in Python 3.4, and a sweep of the library removing anything keep around for Python 2.7 compatibility.