Update typing of append_context

Author: Brian Villemarette

aws_lambda_powertools/event_handler/api_gateway.py

@@ -1360,7 +1360,7 @@ def _reset_processed_stack(self):
         """Reset the Processed Stack Frames"""
         self.processed_stack_frames.clear()
 
-    def append_context(self, **additional_context):
+    def append_context(self, **additional_context) -> None:
         """Append key=value data as routing context"""
         self.context.update(**additional_context)