fix: __aenter__ abstractmethod signature

Author: Quakumei

aio_pika/abc.py

@@ -387,7 +387,7 @@ def __aiter__(self) -> "AbstractQueueIterator":
         raise NotImplementedError
 
     @abstractmethod
-    def __aenter__(self) -> Awaitable["AbstractQueueIterator"]:
+    async def __aenter__(self) -> Awaitable["AbstractQueueIterator"]:
         raise NotImplementedError
 
     @abstractmethod