small fix

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

Author: PlugaruT

src/cloudevents/core/v1/exceptions.py

@@ -42,6 +42,7 @@ class MissingRequiredAttributeError(BaseCloudEventException, ValueError):
     """
 
     def __init__(self, attribute_name: str) -> None:
+        self.attribute_name: str = attribute_name
         super().__init__(f"Missing required attribute: '{attribute_name}'")