improving the error message thrown when attempting to set the value of a constant member

[michalwyszynski93]

The error message is generic and therefore not very useful/actionable for the user who encounters it.
In setattrbehavior.cpp

int
constant_handler( Member* member, CAtom* atom, PyObject* value )
{
    cppy::type_error( "cannot set the value of a constant member" );
    return -1;
}

could be replaced with something like

int
constant_handler( Member* member, CAtom* atom, PyObject* value )
{
    const char* member_str = PyUnicode_AsUTF8(member->name);
    PyObject* object_repr = PyObject_Repr(value);
    const PyObject object_base = atom->ob_base;
    const PyTypeObject* object_type = object_base.ob_type;
    char error_msg[500];
    if((object_repr == NULL) || (object_type == NULL)) {
        snprintf(error_msg, 500, "cannot set the value of a constant member %s", member_str);
        cppy::type_error(error_msg);
        if(object_repr != NULL) {
            Py_DECREF(object_repr);
        }
        return -1;
    }
    const char* value_str = PyUnicode_AsUTF8(object_repr);
    const char* type_str = object_type->tp_name;
    snprintf(error_msg, 500, "cannot set the value %s of a constant member %s of an instance of %s", value_str, member_str, type_str);
    cppy::type_error(error_msg);
    Py_DECREF(object_repr);
    return -1;
}