use PythonPlot for test plots

[juliasloan25]

Purpose

The conservation checker plots are currently failing on MacOS due to an incompatibility with the GR backend introduced in Plots.jl v1.40.0. This PR works around that bug by replacing the GR backend with PythonPlot for this test.

[Sbozzolo]

Can GR be removed?

[juliasloan25]

Can GR be removed?

PythonPlot is only being used in test so we still need GR in the other envs (it's in experiments/AMIP and perf). I don't see GR in test/Project.toml so I think it's removed there. Though I'm not sure why I had to add PythonPlot explicitly if we didn't do that with GR before. If I remove it and run the tests I get:

LoadError: ArgumentError: Package PythonPlot not found in current path.
Run import Pkg; Pkg.add("PythonPlot") to install the PythonPlot package.

[Sbozzolo]

Can GR be removed?

PythonPlot is only being used in test so we still need GR in the other envs (it's in experiments/AMIP and perf). I don't see GR in test/Project.toml so I think it's removed there. Though I'm not sure why I had to add PythonPlot explicitly if we didn't do that with GR before. If I remove it and run the tests I get:

LoadError: ArgumentError: Package PythonPlot not found in current path.
Run import Pkg; Pkg.add("PythonPlot") to install the PythonPlot package.

If GR is incompatible with the latest MacOS, we should use PythonPlot everywhere. I am guessing you have to add PythonPlot but not GR because GR is the default backend (you can see that it is a required dependency in the Project.toml

[juliasloan25]

bug is fixed by GR v0.73.2 (won't merge this)