Logs 'msg % args' with severity 'WARN'.

def warn(self, msg, *args, **kwargs):
    """Logs 'msg % args' with severity 'WARN'."""
    if six.PY3:
      warnings.warn("The 'warn' method is deprecated, use 'warning' instead",
                    DeprecationWarning, 2)
    self.log(logging.WARN, msg, *args, **kwargs)