Update src/graph.rs

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

src/graph.rs

@@ -477,7 +477,7 @@ impl PyGraph {
     #[pyo3(text_signature = "(self, node, /)")]
     pub fn has_node(&self, node: usize) -> bool {
         let index = NodeIndex::new(node);
-        return self.graph.node_weight(index).is_some();
+        self.graph.contains_node(index)
     }
 
     /// Return True if there is an edge between ``node_a`` and ``node_b``.