`HostAxesBase.get_aux_axes` now defaults to using the same base axes class as the host axes

If using an mpl_toolkits.axisartist-based host Axes, the parasite Axes will also be based on mpl_toolkits.axisartist. This behavior is consistent with HostAxesBase.twin, HostAxesBase.twinx, and HostAxesBase.twiny.