Make main() return exit code instead of calling sys.exit()

[jwodder]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.62%. Comparing base (49cbd72) to head (4b60974).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #127   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files          17       17           
  Lines        1263     1264    +1     
  Branches      188      188           
=======================================
+ Hits         1233     1234    +1     
  Misses         15       15           
  Partials       15       15           

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