docs: def kurtosis(x):
      """
      Returns the kurtosis of x (calculated with the adjusted Fisher-Pearson standardized
      moment coefficient G2).
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float
      """
      if not isinstance(x, pd.Series):
          x = pd.Series(x)
      return pd.Series.kurtosis(x)