Use colorcet register_cmap

[hoxbro]

When https://github.com/holoviz/colorcet/pull/90 is merged and released, it will remove the DeprecationWarning.

[codecov-commenter]

Codecov Report

Merging #5460 (7e8c66b) into master (07ef9b3) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5460      +/-   ##
==========================================
+ Coverage   88.09%   88.12%   +0.03%     
==========================================
  Files         301      301              
  Lines       61991    62096     +105     
==========================================
+ Hits        54608    54722     +114     
+ Misses       7383     7374       -9     

Impacted Files	Coverage Δ
holoviews/plotting/mpl/__init__.py	81.61% <100.00%> (-0.14%)	:arrow_down:
holoviews/core/options.py	82.44% <0.00%> (-0.36%)	:arrow_down:
holoviews/__init__.py	72.22% <0.00%> (ø)
holoviews/core/data/pandas.py	93.30% <0.00%> (ø)
holoviews/tests/core/test_apply.py	100.00% <0.00%> (ø)
holoviews/tests/plotting/bokeh/test_sankey.py	100.00% <0.00%> (ø)
...iews/tests/plotting/matplotlib/test_elementplot.py	100.00% <0.00%> (ø)
holoviews/plotting/bokeh/element.py	88.87% <0.00%> (+0.01%)	:arrow_up:
holoviews/core/util.py	86.08% <0.00%> (+0.02%)	:arrow_up:
...loviews/tests/plotting/matplotlib/test_renderer.py	98.70% <0.00%> (+0.02%)	:arrow_up:
... and 4 more

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[philippjfr]

Either need to have a fallback or need to pin the new version of colorcet.

[hoxbro]

The register_cmap function has been in colorcet as a "normal" matplotlib import for a long time, and the updated register_cmap function added in https://github.com/holoviz/colorcet/pull/90 should be backward compatible. So I don't think a pin is necessary, but correct me if I'm wrong.

[philippjfr]

Ah did not know it was already there before the version. Ignore me then.

[jlstevens]

Colorcet has now been released with that PR merged there. Merging here too, thanks!