Message 309618 - Python tracker

I think https://github.com/python/cpython/pull/4458 is ready to go now, but I expect it would benefit from a review before I merge it. If anyone has the time to take a look, it would be much appreciated :)

However, I'd also like to get it in for 3.7.0a4 this week, so I'll merge it tomorrow regardless - if we decide we need to tweak it before the first beta, we can do that.