Use mpl.colormaps.get_cmap for compatibility with matplotlib 3.9

[pllim]

Fix #421 , maybe.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (307a11d) 86.63% compared to head (0175678) 86.61%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
glue_jupyter/state_traitlets_helpers.py	60.00%	2 Missing ⚠️
glue_jupyter/utils.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
- Coverage   86.63%   86.61%   -0.03%     
==========================================
  Files          89       89              
  Lines        5163     5169       +6     
==========================================
+ Hits         4473     4477       +4     
- Misses        690      692       +2     

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[dhomeier]

There are no tests for that functionality which may be the reason we missed the deprecation phase, but I've tested the options for matplotlib 3.3.4-3.9.0dev.
There is one other use in utils.py that should also be updated along with this; I think scalar_to_png_data should support Colormap and thus use get_cmap even more than here. Thanks!

[pllim]

Thanks for the review, @dhomeier ! I think I have applied all your comments. Does it look good now?

[dhomeier]

Looks all good, and thanks for the isort cleanup, too!