Maybe it's possible to give an interpretation to awaiting on a threaded
Future? __await__ could return a new asyncio Future, and add a completion
callback to the original Future that makes the asyncio Future ready and
transfers the result/exception. This would have to use
loop.call_soon_threadsafe() to transfer control from the exector thread to
the thread where the loop is running.

The only thing I don't know is whether it's possible for __await__ to
return a Future.