Fix statsmodels deprecation in demo extension

[daniel-thom]

CI reported this failure when it used statsmodels v0.13.0. Jade now restricts it to v0.12.

    def test_autoregression_analysis(test_data_dir):
        """Should return csv result and png plot"""
        country = "Mock Country"
        data = os.path.join(test_data_dir, "demo", "mock_country.csv")
        output = os.path.join(tempfile.gettempdir(), "jade-unit-test-dir")
        os.makedirs(output, exist_ok=True)
    
>       result_file, plot_file = autoregression_analysis(country, data, output)

tests/unit/extensions/demo/test_autoregression_execution.py:22: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
jade/extensions/demo/autoregression_execution.py:40: in autoregression_analysis
    model = AR(train)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <statsmodels.tsa.ar_model.AR object at 0x7f4837017c70>
args = (array([ 100,  200,  300,  400,  500,  550,  600,  650,  700,  750,  800,
        900, 1000, 1100, 1200, 1300, 1350, 1400, 1450]),)
kwargs = {}

    def __init__(self, *args, **kwargs):
>       raise NotImplementedError(
            "AR has been removed from statsmodels and replaced with "
            "statsmodels.tsa.ar_model.AutoReg."
        )
E       NotImplementedError: AR has been removed from statsmodels and replaced with statsmodels.tsa.ar_model.AutoReg.