Deprecation fix mpl 3.10 and beyond

[cvanelteren]

There are a lot of deprecation warnings from MPL and or Ultraplot existing in the current build. The majority of these fixes can be addressed by modifying the tests. Some of the deprecation warnings ensures keywords of usage to be compatible with mpl 3.10 which will result in errors in later versions.

There are a few errors I have not addressed that warrants some discussion.

• The first one

  ultraplot/tests/test_1dplots.py::test_boxplot_vectors
  /home/casper/micromamba/envs/ultraplot-dev/lib/python3.11/site-packages/pytest_mpl/plugin.py:125: UltraPlotWarning: Passing lists to fillalpha was removed in v0.9. Please specify different opacities using the property cycle colors instead.
      store.return_value[test_name] = obj(*args, **kwargs)

• The second one

  ultraplot/tests/test_subplots.py::test_axis_sharing[labels]
  /home/casper/micromamba/envs/ultraplot-dev/lib/python3.11/site-packages/pytest_mpl/plugin.py:125: UltraPlotWarning: Calling arbitrary axes methods from SubplotGrid was deprecated in v0.8 and will be removed in a future release. Please index the grid or loop over the grid instead.
  store.return_value[test_name] = obj(*args, **kwargs)

For both of these, I don't agree with the deprecation warning. The first issue indicates that we should move towards a cycler object. However, adjusting the subplot to match the target baseline cannot be readily achieved through the cycler object. It requires setting the patches to a different alpha and leaving the lines with alpha = 1. This would (I think) require an extension of what the cycler object can handle which is to set all potential properties of all the artists. I don't think it can currently do this.

The second error implies the removal of multi-axis formatting. Personally, I would be against removing this feature as it is one of the most powerful features of UltraPlot.

[cvanelteren]

Making this a draft as we have not updated to 3.10 yet.

[cvanelteren]

Note I had to rename the warning UltraplotWarning to UltraPlotWarning due to some weird pytest-xdist issue.