REF: Implement isin on DTA instead of DTI
> Why add it to the array class while for none of our other EAs it is implemented on the array?

1) it is implemented on Categorical
2) jreback requested the move in #37528
3) i intend to de-duplicate code in `isin`, `equals`, and `_validate_listlike` OK, sounds good thanks @jbrockmendel 

yeah I am +1 on adding .isin too the base class (and coercion would go thru `_from_sequence` so likely this would mostly be a base-class method (not much need to override)