5.1rc0 released

[Augie Fackler]

Please update your package builds, thanks.

Python 3 support remains basically "beta" quality with this release, but again we'd appreciate testing. Note that if your platform is moving to Python 3 in the next 3 months, you should probably consider packaging with Py3 for this cycle, but we'd like to hear about that _now_ and know in advance we should expect those users.

My hope is that 5.2 is formally supported on Py3.

Release notes should happen soon, I didn't do those just yet. It's a smaller release than 5.0, but still has some good stuff.