add matplotlib as a requirement

[jGaboardi]

This PR resolves #79

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.7%. Comparing base (03ca561) to head (52db923).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main     #80     +/-   ##
=======================================
- Coverage   96.7%   96.7%   -0.0%     
=======================================
  Files          7       7             
  Lines        491     490      -1     
=======================================
- Hits         475     474      -1     
  Misses        16      16             

Files with missing lines	Coverage Δ
inequality/pen.py	89.8% <100.0%> (-0.1%)	⬇️