tests: raise NotImplementedError for custom ext

mypy flagged the super call as a problem b/c it just raised a
NotImplementedError. I think I agree -- it's better to be explicit that our test
case shouldn't call the schema uri.

tests/extensions/test_custom.py

@@ -48,7 +48,7 @@ def add_link(self, target: pystac.STACObject) -> None:
 
     @classmethod
     def get_schema_uri(cls) -> str:
-        return super().get_schema_uri()
+        raise NotImplementedError
 
     @staticmethod
     def custom_ext(obj: T) -> "CustomExtension[T]":