ruff_python_ast: Make Singleton Copy (#14943)

## Summary

Minor changed pulled out from #14759, as it seems to make sense in
isolation.

## Test Plan

—

crates/ruff_python_ast/src/nodes.rs

@@ -4061,7 +4061,7 @@ impl Ranged for Identifier {
     }
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Singleton {
     None,
     True,