docs: def load_class(module_name, class_name):
      """Return class object specified by module name and class name.
  
      Return None if module failed to be imported.
  
      :param module_name: string module name
      :param class_name: string class name
      """
      try:
          plugmod = import_module(module_name)
      except Exception as exc:
          warn("Importing built-in plugin %s.%s raised an exception: %r" %
               (module_name, class_name, repr(exc)), ImportWarning)
          return None
      else:
          return getattr(plugmod, class_name)