remove the use of `pd.Series.append()`

[hbaniecki]

For example:

FutureWarning: The series.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  tpr = pd.Series([0]).append(tpr_temp.y_true.cumsum()) / _df.y_true.sum()
FutureWarning: The series.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  fpr = pd.Series([0]).append(fpr_temp.y_true.cumsum()) / (1 - _df.y_true).sum()