Use anext() instead of __anext__()

`pylint` doesn't like calling dunder methods explicitly and `anext()`
was added in Python 3.10. Since we depend on Python 3.11 we can just use
`anext()` and make `pylint` happy.

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

Author: llucax

src/frequenz/channels/_receiver.py

@@ -230,7 +230,7 @@ async def receive(self) -> ReceiverMessageT_co:  # noqa: DOC503
             ReceiverError: If there is some problem with the receiver.
         """
         try:
-            received = await self.__anext__()  # pylint: disable=unnecessary-dunder-call
+            received = await anext(self)
         except StopAsyncIteration as exc:
             # If we already had a cause and it was the receiver was stopped,
             # then reuse that error, as StopAsyncIteration is just an artifact