Make sender errors type variable covariant

In general we want a `SenderError[list[MoreSpecific]]` to be a subtype
of `SenderError[list[LessSpecific]]`, as it is just a container for the
sender.

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

Author: llucax

src/frequenz/channels/_sender.py

@@ -53,7 +53,7 @@
 from typing import Generic
 
 from ._exceptions import Error
-from ._generic import SenderMessageT_contra
+from ._generic import SenderMessageT_co, SenderMessageT_contra
 
 
 class Sender(ABC, Generic[SenderMessageT_contra]):
@@ -71,13 +71,13 @@ async def send(self, message: SenderMessageT_contra, /) -> None:
         """
 
 
-class SenderError(Error, Generic[SenderMessageT_contra]):
+class SenderError(Error, Generic[SenderMessageT_co]):
     """An error that originated in a [Sender][frequenz.channels.Sender].
 
     All exceptions generated by senders inherit from this exception.
     """
 
-    def __init__(self, message: str, sender: Sender[SenderMessageT_contra]):
+    def __init__(self, message: str, sender: Sender[SenderMessageT_co]):
         """Initialize this error.
 
         Args:
@@ -86,5 +86,5 @@ def __init__(self, message: str, sender: Sender[SenderMessageT_contra]):
                 happened.
         """
         super().__init__(message)
-        self.sender: Sender[SenderMessageT_contra] = sender
+        self.sender: Sender[SenderMessageT_co] = sender
         """The sender where the error happened."""