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 cmocean, like register_cmap

https://github.com/matplotlib/matplotlib/pull/23668

If it's any help, here's the patch I wrote to adapt to these deprecations in yt https://github.com/yt-project/yt/pull/4089

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