Respect static-method binding in builtin __self__ (#9291)

Reuse get_self() so static builtin methods expose None rather than their
stored class receiver, while preserving bound and module receivers.

crates/vm/src/builtins/builtin_func.rs

@@ -195,13 +195,9 @@ impl PyNativeFunction {
     // meth_get__self__ in CPython
     #[pygetset]
     fn __self__(zelf: NativeFunctionOrMethod, vm: &VirtualMachine) -> PyObjectRef {
-        if let Some(bound) = &zelf.0.zelf {
-            return bound.clone();
-        }
-        if let Some(module) = &zelf.0.module_object {
-            return module.clone();
-        }
-        vm.ctx.none()
+        zelf.0
+            .get_self()
+            .map_or_else(|| vm.ctx.none(), ToOwned::to_owned)
     }
 
     // meth_reduce: the name when unbound or bound to a module, otherwise