Fix return annotation of Observable.__await__

This fixes issues with mypy type checking when awaiting an observable

reactivex/observable/observable.py

@@ -3,7 +3,7 @@
 
 import asyncio
 import threading
-from typing import Any, Callable, Iterable, Optional, TypeVar, Union, cast, overload
+from typing import Any, Callable, Generator, Iterable, Optional, TypeVar, Union, cast, overload
 
 from reactivex import abc
 from reactivex.disposable import Disposable
@@ -254,7 +254,7 @@ def run(self) -> Any:
 
         return run(self)
 
-    def __await__(self) -> Iterable[_T]:
+    def __await__(self) -> Generator[Any, None, _T]:
         """Awaits the given observable.
 
         Returns:
@@ -263,7 +263,8 @@ def __await__(self) -> Iterable[_T]:
         from ..operators._tofuture import to_future_
 
         loop = asyncio.get_event_loop()
-        return iter(self.pipe(to_future_(scheduler=AsyncIOScheduler(loop=loop))))
+        future: asyncio.Future[_T] = self.pipe(to_future_(scheduler=AsyncIOScheduler(loop=loop)))
+        return future.__await__()
 
     def __add__(self, other: Observable[_T]) -> Observable[_T]:
         """Pythonic version of :func:`concat <reactivex.concat>`.