add AsyncGenerator to typing.pyi

This parallels python/typing#346

Author: JelleZijlstra

stdlib/3/typing.pyi

@@ -151,6 +151,24 @@ class AsyncIterator(AsyncIterable[_T_co],
     def __anext__(self) -> Awaitable[_T_co]: ...
     def __aiter__(self) -> 'AsyncIterator[_T_co]': ...
 
+if sys.version_info >= (3, 6):
+    class AsyncGenerator(AsyncIterator[_T_co], Generic[_T_co, _T_contra]):
+        @abstractmethod
+        def __anext__(self) -> Awaitable[_T_co]: ...
+
+        @abstractmethod
+        def asend(self, value: _T_contra) -> Awaitable[_T_co]: ...
+
+        @abstractmethod
+        def athrow(self, typ: Type[BaseException], val: Optional[BaseException] = None,
+                   tb: Any = None) -> Awaitable[None]: ...
+
+        @abstractmethod
+        def aclose(self) -> Awaitable[None]: ...
+
+        @abstractmethod
+        def __aiter__(self) -> 'AsyncGenerator[_T_co, _T_contra]': ...
+
 class Container(Generic[_T_co]):
     @abstractmethod
     def __contains__(self, x: object) -> bool: ...