CI FutureWarning: The frame.append method is deprecated

[DimitriPapadopoulos]

pandas.DataFrame.append → pandas.concat

CI warning to fix:

rampwf/hyperopt/tests/test_hyperparameter.py: 256 warnings
  /home/runner/work/ramp-workflow/ramp-workflow/rampwf/hyperopt/hyperopt.py:442: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
    self.df_scores_ = self.df_scores_.append(row, ignore_index=True)

See Don’t append rows to a pandas DataFrame.