Fix #3497: Ignore done tasks when cancels pending activities on web.r…

…un_app finalization.

aiohttp/helpers.py

@@ -63,10 +63,15 @@
     from typing_extensions import ContextManager
 
 
-all_tasks = asyncio.Task.all_tasks
+def all_tasks(
+        loop: Optional[asyncio.AbstractEventLoop] = None
+) -> Set['asyncio.Task[Any]']:
+    tasks = list(asyncio.Task.all_tasks(loop))  # type: ignore
+    return {t for t in tasks if not t.done()}
+
 
 if PY_37:
-    all_tasks = getattr(asyncio, 'all_tasks')  # use the trick to cheat mypy
+    all_tasks = getattr(asyncio, 'all_tasks')  # noqa
 
 
 _T = TypeVar('_T')