ListedColormap: don't pass n colors

[mathause]

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

Should fix the failed upstream dev ci. Let's see if that also works for older versions of matplotlib.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.13%. Comparing base (3f71c4e) to head (eab5c6e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files          15       15           
  Lines         576      576           
=======================================
  Hits          548      548           
  Misses         28       28           

Flag	Coverage Δ
unittests	95.13% <100.00%> (ø)

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