docs: def mark_error_retryable(error):
      """
      Mark an exception instance or type as retryable. If this exception
      is caught by ``pyramid_retry`` then it may retry the request.
  
      """
      if isinstance(error, Exception):
          alsoProvides(error, IRetryableError)
      elif inspect.isclass(error) and issubclass(error, Exception):
          classImplements(error, IRetryableError)
      else:
          raise ValueError(
              'only exception objects or types may be marked retryable')