Use PyUnicode_AsEncodedString() instead of PyUnicode_AsEncodedObject()

python/google/protobuf/pyext/message.cc

@@ -779,7 +779,7 @@ PyObject* CheckString(PyObject* arg, const FieldDescriptor* descriptor) {
       encoded_string = arg;  // Already encoded.
       Py_INCREF(encoded_string);
     } else {
-      encoded_string = PyUnicode_AsEncodedObject(arg, "utf-8", NULL);
+      encoded_string = PyUnicode_AsEncodedString(arg, "utf-8", NULL);
     }
   } else {
     // In this case field type is "bytes".