Update qiskit_addon_obp/backpropagation.py

Co-authored-by: Max Rossmannek <oss@zurich.ibm.com>

qiskit_addon_obp/backpropagation.py

@@ -164,7 +164,7 @@ def handle_timeout(signum, frame):
             # PERF: we will likely need to parallelize this loop
             for i in range(num_observables):
                 non_trivial_slice = False
-                op_nodes = reversed(list(circuit_to_dag(slice_).topological_op_nodes()))
+                op_nodes = reversed(circuit_to_dag(slice_).topological_op_nodes())
                 for op_idx, op_node in enumerate(op_nodes):
                     # Ignore barriers within slices
                     if op_node.name == "barrier":