fixup! ZEO.asyncio: switch to async/await style

@d-maurer notes at #218 (comment) :

    Why do we need this special __await__ logic? It is not present in #195 and nevertheless all uvloop tests passed.

and he is right - in 7f574ec I missed to do the full removal of py2
support code from CoroutineExecutor:

in py2 it was

	@coroutine
	def dothing1():
	    yield dothing2()

this way the yield for dothing2 was working this way:

- dothing2() called - it returns a coroutine object
- yield yields that coroutine object, not objects that second coroutine
  object would yield by itself. This is the difference in beween yield
  and `yield from`.
- CoroutineExecutor was noticing such yield and handling `yield coro()`
  with the same semantic as if it was `yield from coro()` by manually
  creating another AsyncTask.

but on py3 await has semantic of `yield from` and so we do not need to
keep all that `yield from` emulating code. 7f574ec dropped part of
that, but did not drop special handling of `yield coro()` if coro is
non-native custom coroutine function.

Let that code go as well.

src/ZEO/asyncio/_futures.pyx

@@ -272,29 +272,11 @@ cdef class CoroutineExecutor:
                 result._asyncio_future_blocking = False
                 await_next = result
 
+            # bad await
             else:
-                # object with __await__ - e.g. @cython.iterable_coroutine used by uvloop
-                risawaitable = True
-                try:
-                    rawait = result.__await__()
-                except AttributeError:
-                    risawaitable = False
-                else:
-                    # cython.iterable_coroutine returns `coroutine_wrapper` that mimics
-                    # iterator/generator but does not inherit from types.GeneratorType .
-                    await_next = AsyncTask(rawait, self.task.get_loop())
-
-                if not risawaitable:
-                    # bare yield
-                    if result is None:
-                        await_next = Future(self.task.get_loop())
-                        await_next.set_result(None)
-
-                    # bad yield
-                    else:
-                        await_next = Future(self.task.get_loop())
-                        await_next.set_exception(
-                                RuntimeError("Task got bad yield: %r" % (result,)))
+                await_next = Future(self.task.get_loop())
+                await_next.set_exception(
+                        RuntimeError("Task got bad await: %r" % (result,)))
 
             if self.cancel_requested:
                 _cancel_future(await_next, self.cancel_msg)

src/ZEO/asyncio/futures.py

@@ -253,29 +253,11 @@ def step(self):
                 result._asyncio_future_blocking = False
                 await_next = result
 
+            # bad await
             else:
-                # object with __await__ - e.g. @cython.iterable_coroutine used by uvloop
-                risawaitable = True
-                try:
-                    rawait = result.__await__()
-                except AttributeError:
-                    risawaitable = False
-                else:
-                    # cython.iterable_coroutine returns `coroutine_wrapper` that mimics
-                    # iterator/generator but does not inherit from types.GeneratorType .
-                    await_next = AsyncTask(rawait, self.task.get_loop())
-
-                if not risawaitable:
-                    # bare yield
-                    if result is None:
-                        await_next = Future(self.task.get_loop())
-                        await_next.set_result(None)
-
-                    # bad yield
-                    else:
-                        await_next = Future(self.task.get_loop())
-                        await_next.set_exception(
-                                RuntimeError("Task got bad yield: %r" % (result,)))
+                await_next = Future(self.task.get_loop())
+                await_next.set_exception(
+                        RuntimeError("Task got bad await: %r" % (result,)))
 
             if self.cancel_requested:
                 _cancel_future(await_next, self.cancel_msg)