fix for Python 3.8: asyncio.Task is not a generic type

Signed-off-by: Achille Roussel <achille.roussel@gmail.com>

src/dispatch/test/__init__.py

@@ -83,7 +83,7 @@ def url(self):
 
 
 class Service(web.Application):
-    tasks: Dict[str, asyncio.Task[CallResult]]
+    tasks: Dict[str, asyncio.Task]
     _session: Optional[aiohttp.ClientSession] = None
 
     def __init__(self, session: Optional[aiohttp.ClientSession] = None):