Silence mypy error in AbstractRoute ctor (aio-libs#3927)

- Don't reuse variable to decomplicate logic
- Silence error at return line as mypy cannot deduce that result
  cannot be Awaitable after the 'if'

aiohttp/web_urldispatcher.py

@@ -155,8 +155,8 @@ def __init__(self, method: str,
             async def handler_wrapper(request: Request) -> StreamResponse:
                 result = old_handler(request)
                 if asyncio.iscoroutine(result):
-                    result = await result
-                return result
+                    return await result
+                return result  # type: ignore
             old_handler = handler
             handler = handler_wrapper