Add Async classes to typing stub.

Author: Roy Williams

stdlib/3/typing.pyi

@@ -87,14 +87,27 @@ class Hashable(metaclass=ABCMeta):
     @abstractmethod
     def __hash__(self) -> int: ...
 
+class Awaitable(Generic[_T_co]):
+    @abstractmethod
+    def __await__(self) -> Iterator[_T_co]:...
+
+class AsyncIterable(Generic[_T_co]):
+    @abstractmethod
+    def __anext__(self) -> Awaitable[_T_co]:...
+
+class AsyncIterator(AsyncIterable[_T_co], Generic[_T_co]):
+    @abstractmethod
+    def __anext__(self) -> Awaitable[_T_co]:...
+    def __aiter__(self) -> 'AsyncIterator'[_T_co]:...
+
 class Iterable(Generic[_T_co]):
     @abstractmethod
     def __iter__(self) -> Iterator[_T_co]: ...
 
 class Iterator(Iterable[_T_co], Generic[_T_co]):
     @abstractmethod
     def __next__(self) -> _T_co: ...
-    def __iter__(self) -> Iterator[_T_co]: ...
+    def __iter__(self) -> 'Iterator'[_T_co]: ...
 
 class Container(Generic[_T_co]):
     @abstractmethod