TST: clean up deprecation warning
I think these are only in 3.7, but have to be careful not to break other versions.

```
(pandas) bash-3.2$ pytest pandas/tests/dtypes/test_dtypes.py -k test_pickle
========================================================================================================================= test session starts ==========================================================================================================================
platform darwin -- Python 3.7.2, pytest-4.0.2, py-1.7.0, pluggy-0.8.0
hypothesis profile 'ci' -> timeout=-1, deadline=500, suppress_health_check=[HealthCheck.too_slow], database=DirectoryBasedExampleDatabase('/Users/jreback/pandas/.hypothesis/examples')
rootdir: /Users/jreback/pandas, inifile: setup.cfg
plugins: xdist-1.25.0, forked-0.2, cov-2.6.0, hypothesis-3.85.3
collected 244 items / 240 deselected                                                                                                                                                                                                                                   

pandas/tests/dtypes/test_dtypes.py ....                                                                                                                                                                                                                          [100%]

=========================================================================================================================== warnings summary ===========================================================================================================================
pandas/tests/dtypes/test_dtypes.py::TestCategoricalDtype::test_pickle
  /Users/jreback/miniconda3/envs/pandas/lib/python3.7/site-packages/botocore/vendored/requests/packages/urllib3/_collections.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping, MutableMapping
  /Users/jreback/miniconda3/envs/pandas/lib/python3.7/site-packages/botocore/vendored/requests/packages/urllib3/_collections.py:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Mapping, MutableMapping

-- Docs: https://docs.pytest.org/en/latest/warnings.html
========================================================================================================= 4 passed, 240 deselected, 2 warnings in 0.25 seconds =========================================================================================================
```