Move mark registration
Should avoid warnings like

```
  D:\a\1\s\test_venv\lib\site-packages\pandas\tests\plotting\test_series.py:655: 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

```

in https://dev.azure.com/pandas-dev/pandas-wheels/_build/results?buildId=38672&view=logs&j=c0130b29-789d-5a3c-6978-10796a508a7f&t=e120bc6c-1f5e-5a41-8f0a-1d992cd2fbfb
lgtm thanks @TomAugspurger 