PyO3 Python callback from Rust

lib.err

error[E0277]: the trait bound `pyo3::object::PyObject: pyo3::instance::PyNativeType` is not satisfied
  --> src\lib.rs:16:13
   |
16 |             ObjectProtocol::is_callable(&callback)
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `pyo3::instance::PyNativeType` is not implemented for `pyo3::object::PyObject`
   |
   = note: required because of the requirements on the impl of `pyo3::objectprotocol::ObjectProtocol` for `pyo3::object::PyObject`
   = note: required by `pyo3::objectprotocol::ObjectProtocol::is_callable`

lib.rs

use pyo3::exceptions;
use pyo3::prelude::*;
#[pymodule]
fn py_test(py: Python, m: &PyModule) -> PyResult<()> {
    #[pyfn(m, "sum_as_string")]
    fn sum_as_string(py: Python, a: usize, b: usize, callback: PyObject) -> PyResult<String> {
        println!("Hello from Rust!");
        println!("callback is: {:?}", callback);
        println!(
            "callback is: {:?}",
            ObjectProtocol::is_callable(&callback)
        );
        println!("Raising exception...");
        Ok("bye!".to_string())
    }
    Ok(())
}

test.py

import py_test
print(repr(py_test.sum_as_string(1,2,print)))