Use Iterable hint instead of Sequence

Given that the `FrozenList` type hint is `Union[List[_T], Iterable[_T]]`
the Application middleware type should not restrict to `Sequence` and
support all `Iterable` types.

aiohttp/web_app.py

@@ -75,7 +75,7 @@ class Application(MutableMapping[str, Any]):
 
     def __init__(self, *,
                  logger: logging.Logger=web_logger,
-                 middlewares: Sequence[_Middleware]=(),
+                 middlewares: Iterable[_Middleware]=(),
                  handler_args: Mapping[str, Any]=None,
                  client_max_size: int=1024**2,
                  debug: Any=...  # mypy doesn't support ellipsis