The docs were ambiguous about whether you pass in a coroutine function
or a coroutine object, e.g. is it:

  aestack.push_async_exit(some_async_func)

or

  aestack.push_async_exit(some_async_func())

(It's the first one.)
(cherry picked from commit a3c88ef1)
Co-authored-by: Nathaniel J. Smith <njs@pobox.com>