ListedColormap: repeat colors if necessary

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.md

Apply findings from pydata/xarray#9811

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.15%. Comparing base (cff3381) to head (393c4a5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
+ Coverage   95.13%   95.15%   +0.01%     
==========================================
  Files          15       15              
  Lines         576      578       +2     
==========================================
+ Hits          548      550       +2     
  Misses         28       28              

Flag	Coverage Δ
unittests	95.15% <100.00%> (+0.01%)	⬆️

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