fix: missing type

Signed-off-by: Tudor Plugaru <plugaru.tudor@protonmail.com>

Author: PlugaruT

src/cloudevents/core/v1/event.py

@@ -64,7 +64,7 @@ def _validate_attribute(attributes: dict[str, Any]) -> None:
 
         See https://github.com/cloudevents/spec/blob/main/cloudevents/spec.md#required-attributes
         """
-        errors: dict[str, list] = defaultdict(list)
+        errors: dict[str, list[BaseCloudEventException]] = defaultdict(list)
         errors.update(CloudEvent._validate_required_attributes(attributes))
         errors.update(CloudEvent._validate_optional_attributes(attributes))
         errors.update(CloudEvent._validate_extension_attributes(attributes))