ABCClasses for DatetimeArray etc?
There are a handful of places in the code where we check for for `isinstance(other, ABCDatetimeIndex)` or `getattr(other, '_typ', None) == "datetimeindex"` (and analogous places for TDI, PI).  In most of these cases DatetimeArray/TimedeltaArray/PeriodArray should be handled symmetrically.  Thoughts on what `_typ`s to give them?