CategoricalDtype follows after #16015
- use ``display.max_categories`` in ``CagetegoricalDtype.__unicode__`` [here](https://github.com/pandas-dev/pandas/pull/16015/files#r140619601)
- use ``display.max_categories`` in ``Categorical.__unicode__``
- deprecate usage of ``fastpath`` in ``Categorical`` constructor (#17562)
- deprecate ``.astype(..., categories=)`` in favor of ``CategoricalDtype`` (https://github.com/pandas-dev/pandas/issues/17636)
- move hash type routines to ``hashing.py`` [here](https://github.com/pandas-dev/pandas/pull/16015/files#r137931284)
- validate that pyarrow is compat with changes