add missing return type annotation for __str__ in SFG

b_asic/signal_flow_graph.py

@@ -156,7 +156,7 @@ class SFG(AbstractOperation):
                 if signal.source.operation not in self._original_components_to_new:
                     self._add_operation_connected_tree_copy(signal.source.operation)
     
-    def __str__(self):
+    def __str__(self) -> str:
         """Get a string representation of this SFG."""
         output_string = ""
         for component in self._components_ordered: