Add plotting tests

[keithchev]

This PR adds integration-like tests that generate realistic plots of various common kinds. They are derived from the notebooks in plot_testing/ and are intended to replace them (so this PR also deletes the notebooks).

This PR also adds a pytest CLI option to save the PDF files generated by these tests in a user-specified directory. This allows for manual inspection of the plots generated by the tests. See the README for details.

Drive-bys

• fix a bug introduced in #26 in the colormap registration function.
• disallow matplotlib 3.8.0 because it has a bug that breaks apc.mpl.setup.
• disallow matplotlib 3.9 because it introduces breaking API changes.
• add missing dev deps (seaborn, sklearn, pandas) (addresses #23).
• update the poetry lockfile.

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[keithchev]

One higher-level question: should we remove the Jupyter notebooks? The goal of those notebooks is to serve as example code for folks to be able to understand how the library can be used in different contexts. Replacing those entirely with the tests prevents users from being able to have more interpretable examples to look at. We don't have to keep those exact notebooks or put them in the same place, but I do think it would be good for us to provide some copypaste-able code for this.

Yeah, makes sense. For now I've added the notebooks back into the plot_testing/ directory but we can move these to a more appropriately named directory later on.