Make the mapping_function to Receiver.map() positional-only

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/_receiver.py

@@ -225,7 +225,7 @@ async def receive(self) -> _T_co:
             raise ReceiverStoppedError(self) from exc
         return received
 
-    def map(self, mapping_function: Callable[[_T_co], _U_co]) -> Receiver[_U_co]:
+    def map(self, mapping_function: Callable[[_T_co], _U_co], /) -> Receiver[_U_co]:
         """Apply a mapping function on the received message.
 
         Tip: