License moved to PSF v2

[brettcannon]

I realized having a little script to help with building Python would be helpful here, but that CC0 is not really set up for code. So I turned on the CLA bot and set the PSF v2 license to go into effect starting today (CC0 still applies until today). To avoid any "what does 'until' mean, I won't be merging any code until tomorrow.