Add sys.{get,set}_asyncgen_hooks (#4195)

stdlib/3/sys.pyi

@@ -214,3 +214,12 @@ if sys.version_info >= (3, 8):
     unraisablehook: Callable[[UnraisableHookArgs], Any]
     def addaudithook(hook: Callable[[str, Tuple[Any, ...]], Any]) -> None: ...
     def audit(__event: str, *args: Any) -> None: ...
+
+if sys.version_info >= (3, 6):
+    from typing import AsyncGenerator
+    _AsyncgenHook = Optional[Callable[[AsyncGenerator[Any, Any]], None]]
+    class _asyncgen_hooks(Tuple[_AsyncgenHook, _AsyncgenHook]):
+        firstiter: _AsyncgenHook
+        finalizer: _AsyncgenHook
+    def get_asyncgen_hooks() -> _asyncgen_hooks: ...
+    def set_asyncgen_hooks(firstiter: _AsyncgenHook = ..., finalizer: _AsyncgenHook = ...) -> None: ...