v0.5.6

This is a minor release pre-empting some upstream deprecations in numpy and pandas.

• Avoid using deprecated is_categorical_dtype from pandas at all in pandas >=0.25. (thanks @EpigeneMax, @lesteve, @bashtage)
• Avoid using deprecated numpy.row_stack in favor of numpy.vstack in preparation from numpy 2.0. (thanks @bashtage)