pandas.test() produces many PytestUnknownMarkWarning
For example

```python
>>> import pandas
>>> pandas.test(['-k', 'TestPDApi'])
```

Has many warnings like

```
Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/indexing/test_datetime.py:121
  /Users/taugspurger/Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/indexing/test_datetime.py:121: PytestUnknownMarkWarning: Unknown pytest.mark.slow - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    @pytest.mark.slow

Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/methods/test_rank.py:559
  /Users/taugspurger/Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/methods/test_rank.py:559: PytestUnknownMarkWarning: Unknown pytest.mark.single - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    @pytest.mark.single

Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/methods/test_rank.py:560
  /Users/taugspurger/Envs/dask-dev/lib/python3.7/site-packages/pandas/tests/series/methods/test_rank.py:560: PytestUnknownMarkWarning: Unknown pytest.mark.high_memory - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    @pytest.mark.high_memory
```

We register these: https://github.com/pandas-dev/pandas/blob/5da500af76d0031021a8e906d8f34dc3e6ded234/setup.cfg#L56-L62. But the setup.cfg is perhaps not included with the distribution, and so the registered marks aren't available perhaps? Is there anyone more familiar with pytest who knows what we should be doing here?