CategoricalBlock.array_dtype out of date?
`CategoricalBlock.array_dtype` returns `np.object_`, but the docstring describes `array_dtype` as "the dtype to return if I want to construct this block as an array".  Shouldn't the array constructed now be `Categorical`?  In which case, shouldn't the dtype here be `self.dtype`?  (which matches the base class, and in fact makes the attribute unnecessary)