Matplotlib deprecation warnings

[christopherlovell]

On matplotlib 3.7 I am getting the following errors when importing and using cmasher

MatplotlibDeprecationWarning: The get_cmap function was deprecated in Matplotlib 3.7 and will be removed two minor releases later. 
Use ``matplotlib.colormaps[name]`` or ``matplotlib.colormaps.get_cmap(obj)`` instead.
cmap = mplcm.get_cmap(cmap)

And the following:

MatplotlibDeprecationWarning: The register_cmap function was deprecated in Matplotlib 3.7 and will be removed two minor releases later. 
Use ``matplotlib.colormaps.register(name)`` instead.
mplcm.register_cmap(cmap=cmap_mpl_r)

These methods should be updated as described.

[neutrinoceros]

This looks like a dup' of #43
FYI the problem is already addressed on the dev branch, what's missing is a release.

[neutrinoceros]

Also see #79 and #74 .
I'm preparing a new release for which this problem is the main motivation.

[neutrinoceros]

This is now resolved with the release of CMasher 1.7
see https://github.com/1313e/CMasher/releases/tag/v1.7.0