Undo pmdarima hotfix and avoid pmdarima 1.8.1

[aaronreidsmith]

Reference Issues/PRs

Reverts hotfix from #828 (also see alkaline-ml/pmdarima#423) and pins so pmdarima 1.8.1 is avoided.

What does this implement/fix? Explain your changes.

Removes the need for the hotfix and keeps numpy compatibility.

Does your contribution introduce a new dependency? If yes, which one?

No, just fixes an existing one 🙂

What should a reviewer concentrate their feedback on?

N/A

Any other comments?

Sorry I pushed a bad pmdarima version yesterday 😅

PR checklist

For all contributions

• I've added myself to the list of contributors.
• Optionally, I've updated sktime's CODEOWNERS to receive notifications about future changes to these files.
• I've added unit tests and made sure they pass locally.

For new estimators

• I've added the estimator to the online documentation.
• I've updated the existing example notebooks or provided a new one to showcase how my estimator works.

[mloning]

Thanks @aaronreidsmith - much appreciated!