message: fix to build kafka extension under python3
diff: @@ -235,7 +235,7 @@ class FieldList:
  return ', '.join(init_list)
  def field_count(self):
  - return len(self.used_fields())
  + return len(list(self.used_fields()))
  def example_value(self):
  return ', '.join(map(lambda x: x.example_value_for_test(self.version), self.used_fields()))
  