As a mostly-lurker (brought here by Thomas’s call for input), here’s my two cents at a high level: it’s clean and simple, and the proposed syntax is unlikely to ever be used for anything else.
As for the “one obvious way to do it” complaint immediately above, I would argue this proposal is the obvious way – it closely parallels how type variables and generics are done in many other languages, and frankly when I started doing static typing in my own code I was surprised that this wasn’t supported, and found the idea of subclassing Generic and manually instantiating TypeVars to be a kludge.
Basically, it just makes sense to me. I haven’t drilled down into details, but the overall syntax idea is a +1 from me.