Adapt to `statsmodels.tsa.arima_model.ARIMA` deprecation.

[csala]

statsmodels release v0.12.0 deprecated the statsmodels.tsa.arima_model.ARIMA model in favor of statsmodels.tsa.arima.model.ARIMA:

https://github.com/statsmodels/statsmodels/blob/04f00006a7aeb1c93d6894caa420698400da6c33/statsmodels/tsa/arima_model.py#L35

We should adapt to this change before they release v0.13.0, which will fully remove the arima_model module:

MLPrimitives/mlprimitives/adapters/statsmodels.py

Line 2 in 97ab03f

from statsmodels.tsa import arima_model

[sarahmish]

Solved in #279