Update to work with matplotlib=3.6

[hoxbro]

Fixes #89

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.84%. Comparing base (3cdbb67) to head (f8a689b).

Files with missing lines	Patch %	Lines
colorcet/__init__.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   85.78%   85.84%   +0.05%     
==========================================
  Files           8        8              
  Lines        1914     1922       +8     
==========================================
+ Hits         1642     1650       +8     
  Misses        272      272              

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[jbednar]

Thanks!