updated the return statement of is_brace()

Author: janmenjayap

src/sage/graphs/matching_covered_graph.py

@@ -2693,7 +2693,7 @@ def dfs(v, visited, neighbor_iterator):
                     for u, v, w in self.edge_iterator()
                     if (u in X) ^ (v in X)]
 
-                return False, C, X
+                return (False, C, set(X))
 
         return (True, None, None) if coNP_certificate else True