Add Support for Python 3.7

[levlaz]

Python 3.7 was released yesterday. As folks start moving over to this version it would be nice to stay ahead of things and add proper support.

[eli-darkly]

We've added the prerelease 3.7-rc version to our CI builds and it seems to work fine. There isn't a Docker image available yet for the released 3.7, but as soon as there is, we'll start using that.

[eli-darkly]

The CI tests are now running with the official 3.7 release, and I've also manually tested with 3.7 from the command line. Looks good.