refactor: UsageKeyV2Serializer should be a BaseSerializer

...not a Serializer, as it overrides to_representation to a str rather
than a dictionary.

This type error arose during the djangorestframework-stubs upgrade in
the previous commits.

openedx/core/djangoapps/content_libraries/serializers.py

@@ -276,7 +276,7 @@ class ContentLibraryCollectionUpdateSerializer(serializers.Serializer):
     description = serializers.CharField(allow_blank=True)
 
 
-class UsageKeyV2Serializer(serializers.Serializer):
+class UsageKeyV2Serializer(serializers.BaseSerializer):
     """
     Serializes a UsageKeyV2.
     """