pytest deprecation warning
```python
home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/mark.py:73: 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)
```

```
$ ag "pytest.mark.*\(.*\)\("
pandas/tests/test_base.py
1220:    pytest.mark.xfail(reason='PeriodArray not implemented')((
```

though there are probably others.