Add missing Optional keyword to SFG.py

b_asic/signal_flow_graph.py

@@ -9,7 +9,7 @@ from b_asic.signal import Signal, SignalSource, SignalDestination
 from b_asic.simulation import SimulationState, OperationState
 from b_asic.graph_id import GraphIDGenerator, GraphID
 
-from typing import List, Dict, Union
+from typing import List, Dict, Union, Optional
 
 class SFG(BasicOperation):
 	"""