CI: Simplify Cython test coverage

[bashtage]

Simplify Cython coverage run for speed to avoid timeouts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (7f36402) to head (0d07e91).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
- Coverage   99.59%   99.59%   -0.01%     
==========================================
  Files          79       79              
  Lines       18172    18154      -18     
  Branches     1492     1492              
==========================================
- Hits        18099    18081      -18     
  Misses         40       40              
  Partials       33       33              

Flag	Coverage Δ
adder	99.56% <100.00%> (-0.01%)	⬇️
subtractor	99.56% <100.00%> (-0.01%)	⬇️

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