ENH: unique/factorize preserve non-nano
Put the whatsnew note in 1.6, assuming that will be changed to 2.0.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Closing as blocked by #50690 (or reso inference in array_to_datetime)