REF: lazify relativedelta imports
In most cases we don't need it, so delay importing it until we do.

in io.stata it can be replaced with datetime.timedelta.  In the plotting code I _think_ it can be replaced with `tot_secs = (dmax-dmin).total_seconds()`, need to track down why that wasn't done in the first place.
Updated to intentionally raise > In the plotting code I _think_ it can be replaced with `tot_secs = (dmax-dmin).total_seconds()`, need to track down why that wasn't done in the first place.

the original comes from https://github.com/pandas-dev/pandas/pull/1616/commits/d0d15b0fa69a0d464a939b757a104d57fc7b4d58, but there's no context and looks like an initial oversight to me Thanks @jbrockmendel 