Future incompatibility with MPL

[neutrinoceros]

Just wanted to warn you about the pending deprecation of some functions in the matplotlib.cm namespace that are used in cmasher, like get_cmap

matplotlib/matplotlib#23668

If it's any help, here's the patch I wrote to adapt to these deprecations in yt
yt-project/yt#4089

I can also submit a PR here if you lack time for this.

[1313e]

@neutrinoceros Thanks for the heads up.
I will make the changes for this soon.

[1313e]

@neutrinoceros Well, I said I would do it soon, but it seems I cannot find the time to sit down and do this.
So, if you wish, you can open a PR with the required changes for it.

[blakeNaccarato]

I opened another PR against this issue to resolve DeprecationWarnings involving the use of mplcm.get_cmap and mplcm.register_cmap. I made the minimal necessary changes in code and tests. I left CMasher's mirror implementations for getting and registering colormaps untouched, and just changed the underlying calls to non-deprecated approaches.

EDIT: See my latest comment in my PR for details, but I realize this was duplicating work in the PR merged out of master.