BUG: fix compatibility with matplotlib 3.5

[neutrinoceros]

cherry picked from #86

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f949dc7) 99.82% compared to head (69e8c42) 99.30%.

Files	Patch %	Lines
cmasher/utils.py	86.67%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #89      +/-   ##
==========================================
- Coverage   99.82%   99.30%   -0.53%     
==========================================
  Files           4        4              
  Lines         562      568       +6     
==========================================
+ Hits          561      564       +3     
- Misses          1        4       +3     

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