Modernize indexes.timedeltas, indexes.datetimeindex
Two things here.  They should be separate PRs, but so it goes.

indexes.datetimes and indexes.periods use a _field_accessor pattern to define several properties in their index classes.  This updates TimedeltaIndex to use the same pattern, getting rid of the `_get_field` method that was used before.

indexes.datetimes has a _utc() function that I think is designed to lazily import pytz.  That ship has sailed elsewhere, so this removes that extra call.
