Function extract_receiver 

pub fn extract_receiver<'a, 'py, T, R>(
    bound_ref: &'a Bound<'py, PyAny>,
) -> PyResult<R>
where
    T: PyTypeCheck + 'a,
    R: TryFrom<&'a Bound<'py, T>>,
    R::Error: Into<PyErr>,

As extract_receiver_trusted, but performs a checked cast.