Change returned model in `get_model` of `HoltWintersModel`, `HoltModel`, `SimpleExpSmoothingModel`

[Mr-Geekman]

Before submitting (must do checklist)

• [x] Did you read the contribution guide?
• [x] Did you update the docs? We use Numpy format for all the methods and classes.
• [x] Did you write any new necessary tests?
• [x] Did you update the CHANGELOG?

Proposed Changes

Look at #977.

Closing issues

Closes #977.

[github-actions[bot]]

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