Experiment: Convert .gitignore entries to radon ignore/exlude

[devdanzin]

This PR addresses a TODO in build.py: "Convert .gitignore to radon ignore patterns to make the build more efficient". However, timings show no difference in performance even with a lot of added Python files in ignored directories. That happens because radon is called only with modified files from given revisions as targets, so the build is already optimal in this sense.

This PR serves only to document this exploration. Leaving it open for a while in case we can identify a scenario it would help with.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.52%. Comparing base (acf39d2) to head (7959156).
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   95.46%   95.52%   +0.05%     
==========================================
  Files          26       26              
  Lines        1346     1364      +18     
  Branches      287      291       +4     
==========================================
+ Hits         1285     1303      +18     
  Misses         37       37              
  Partials       24       24              

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[tonybaloney]

lmk when this is ready to review