clear up test_from_py_with

tests/test_frompyobject.rs

@@ -300,17 +300,13 @@ fn test_err_rename() {
     );
 }
 
-fn to_string(obj: &PyAny) -> PyResult<String> {
-    Ok(obj.str()?.to_str()?.to_string())
-}
-
 #[derive(Debug, FromPyObject)]
 pub struct Zap {
     #[pyo3(item)]
     name: String,
 
-    #[pyo3(from_py_with = "to_string", item("my_object"))]
-    some_object: String,
+    #[pyo3(from_py_with = "PyAny::len", item("my_object"))]
+    some_object_length: usize,
 }
 
 #[test]
@@ -327,6 +323,6 @@ fn test_from_py_with() {
         let zap = Zap::extract(py_zap).unwrap();
 
         assert_eq!(zap.name, "whatever");
-        assert_eq!(zap.some_object, "[1, 2, 3]");
+        assert_eq!(zap.some_object_length, 3usize);
     });
 }