Deprecation of the top-level cmap registration and access functions in mpl.cm in plot_utils.py from matplotlib 3.9.0

[jcesardasilva]

Hi everyone,

It has been a while since Matplotlib has been informing us about the deprecation of the top-level camp registration and access functions in mpl.cm, but now, in version Matplotlib 3.9.0, I noticed that the functions have definitely changed and it is no longer a simple deprecation warning. More details here: https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.9.0.html#top-level-cmap-registration-and-access-functions-in-mpl-cm

Since ptypy uses them often in plot_utils.py, I don't know what you plan to do.
I made the changes that worked for me, but I will let you check them. If I understood it correctly, those changes should be compatible with Matplotlib>3.6.0 from 2022, so if we implement them, ptypy should not be used with older versions of Matplotlib.

Please let me know what you decide. You can decline this PR if you want.

[jcesardasilva]

Sorry, I did it on the master branch and should have done it in the dev branch, correct? I noticed that dev branch was some commits behind the master, but I did not want to make the rebase of it, which I Iet for the superdeveloppers @daurer and @bjoernenders