Upgrade to python 3.6

[matt-gardner]

I think the main benefit to this is the ability to give type annotations on variables, instead of just methods and method arguments. What needs to be done is just updating all of the continuous integration services / build tools to use the new python version, and make sure everything still works.

[matt-gardner]

I saw a stack trace that made me think this might not be so easy. Some of the libraries we depend on might not be compatible with python 3.6. In particular, I'm guessing that overrides isn't. We'd probably need to patch it and submit a PR here before we can actually migrate ourselves.

[nelson-liu]

Importantly, tensorflow isn't either.

[nelson-liu]

Whoops, never mind looks like you can still install it, but you have to provide a link to a wheel instead of just pip install tensorflow