Describe the feature you want to add to this project

pmdarima is blocking 3.13 compatibility, but is imported only in two places for ndiffs, nsdiffs.

Describe your proposed solution

I suggest to isolate the soft dependency in the code base.

I think the two imports could be moved inside the functions. We should then run tests to see what happens.

After that, we could remove it from the core dep set.

Perhaps a deprecation period is in order, i.e., isolate and then have any call to the function produce a warning, that this feature will require the timeseries dep set from next MINOR.

Describe alternatives you've considered, if relevant

No response

Additional context

No response