TST: pytest deprecation warnings
xref https://travis-ci.org/pandas-dev/pandas/jobs/262021774

lots of these, looks like pytest 3.2 triggers these.

```
/home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/mark.py:66: RemovedInPytest4Warning: Applying marks directly to parameters is deprecated, please use pytest.param(..., marks=...) instead.
For more details, see: https://docs.pytest.org/en/latest/parametrize.html
  warnings.warn(MARK_PARAMETERSET_UNPACKING)
```