docs: add python snippet for "Create the time series model" section of the Forecast a single time series with a univariate model tutorial

[rey-esp]

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Thanks! LGTM, assuming samples tests pass.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/create_single_timeseries_forecasting_model_test.py:43, tag bigquery_dataframes_single_timeseries_forecasting_model_tutorial_create

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