Fix return values of athrow and aclose. (#845)

I misread the PEP here (https://www.python.org/dev/peps/pep-0525/#asynchronous-generator-object); both of these return the yielded type.

Author: JelleZijlstra

stdlib/3/typing.pyi

@@ -161,10 +161,10 @@ if sys.version_info >= (3, 6):
 
         @abstractmethod
         def athrow(self, typ: Type[BaseException], val: Optional[BaseException] = None,
-                   tb: Any = None) -> Awaitable[None]: ...
+                   tb: Any = None) -> Awaitable[_T_co]: ...
 
         @abstractmethod
-        def aclose(self) -> Awaitable[None]: ...
+        def aclose(self) -> Awaitable[_T_co]: ...
 
         @abstractmethod
         def __aiter__(self) -> 'AsyncGenerator[_T_co, _T_contra]': ...