Explicitly set dtype of np.lexsort in group_rank
xref #19481

I didn't see any other instance in `group_rank` where the dtype was open to interpretation, so I'm wondering if `np.lexsort` is returning a plain `int` on 32 bit systems. Hoping that explicitly getting a view of that `np.lexsort` result to match the dtype of `_as` will resolve the test issues