BUG/DEPR: Timestamp/Timedelta resolution
Timedelta.resolution used to return a string, but that was deprecated in 0.25.0.  Enforce that deprecation to return Timedelta(nanoseconds=1), mirroring the stdlib timedelta.resolution behavior.

Timestamp.resolution correctly mirrors datetime.resolution by returning Timedelta(nanoseconds=1), but it is a property whereas for the stdlib it is a class attribute.  This PR makes it a class attribute for Timestamp.