Update MultiIndex checks
cc @WillAyd I'm holding off on most non-trivial groupby stuff, LMK if this is a problem.
@jreback @WillAyd gentle ping for this and #29456.  Neither is important on its own, but both will have merge conflicts with more-important PRs on their way.  can we completely remove _has_complex_internals? also IIRC there is an open PR about removing it (should close). > can we completely remove _has_complex_internals? also IIRC there is an open PR about removing it (should close).

I pushed a rebase on to that this morning.  After this I'll probably have to rebase it again and thatll remove _has_complex_internals entirely. lgtm. merge master once again as I *think* we merged something related to this in last 4 hours. rebased+green thxs