⚠️ upstream-dev CI: `test_PerfectModelEnsemble_plot_bootstrapped_skill_over_leadyear` yerr negative

[Workflow Run URL](https://github.com/pangeo-data/climpred/actions/runs/3170476709)
<details><summary>Python 3.10 Test Summary</summary>

```
climpred/tests/test_graphics.py::test_PerfectModelEnsemble_plot_bootstrapped_skill_over_leadyear: ValueError: 'yerr' must not contain negative values
```

</details>
