Derive _RegistryReceiverFetcher from Generic[T]

Because `ReceiverFetcher` is a `Protocol` type,
`_RegistryReceiverFetcher` will fit as a `ReceiverFetcher` without
having to derive from it.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/actor/_channel_registry.py

@@ -84,7 +84,7 @@ async def _close_channel(self, key: str) -> None:
 T = typing.TypeVar("T")
 
 
-class _RegistryReceiverFetcher(ReceiverFetcher[T]):
+class _RegistryReceiverFetcher(typing.Generic[T]):
     """A receiver fetcher that is bound to a channel registry and a key."""
 
     def __init__(