Type hint. Again a test for Appveyor rolling builds.

Author: PGijsbers

openml/flows/flow.py

@@ -390,7 +390,7 @@ def publish(self, raise_error_if_exists: bool = False) -> 'OpenMLFlow':
                              (flow_id, message))
         return self
 
-    def get_structure(self, key_item):
+    def get_structure(self, key_item: str) -> Dict[str, List[str]]:
         """
         Returns for each sub-component of the flow the path of identifiers
         that should be traversed to reach this component. The resulting dict