> I started a local PR at https://github.com/ncoghlan/cpython/pull/1/files to explore what this might look like in practice.

Looks good to me.


> I'm less sure about __delegated_to__/__returns_to__, since we don't have *any* code in the standard library that reads gi_yieldfrom, and the only code that reads cr_await is a Python 3.5 compatibility hack in asyncio.

I'm -1 on this too.  gi_yieldfrom and cr_await are very special and rarely used things.  Refactoring or unifying them isn't trivial and ultimately not as useful.