update return type to Sequence

Author: ohmayr

packages/gapic-generator/gapic/schema/wrappers.py

@@ -2278,7 +2278,7 @@ def names(self) -> FrozenSet[str]:
         return frozenset(answer)
 
     @utils.cached_property
-    def resource_messages(self) -> FrozenSet[MessageType]:
+    def resource_messages(self) -> Sequence['MessageType']:
         """Returns all the resource message types used in all
         request and response fields in the service."""
 
@@ -2317,7 +2317,7 @@ def gen_indirect_resources_used(message):
         )
 
         sorted_messages = sorted(
-            unique_messages, 
+            unique_messages,
             key=lambda m: m.resource_type_full_path or m.name
         )