tests: Account for argparse bug fix in Python 3.13

[DemiMarie]

Test test_004_enable_opt_mixed is marked as xfail because of an argparse bug, but the bug is fixed in Python 3.13 so the test unexpectedly succeeds. Remove the @unittest.expectedFailure, skip it on Python<3.13, and add a new tests for older Python that checks for the previous buggy behavior.

[marmarek]

CI says "syntax error"

[DemiMarie]

I did test these changes locally, but I also used black to try to format the new code properly, and that required running git add -p or similar. I suspect that while the code I had checked out was fine, the code I committed and pushed was broken. Should be fixed now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.24%. Comparing base (3b9da29) to head (6461531).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #331   +/-   ##
=======================================
  Coverage   75.24%   75.24%           
=======================================
  Files          52       52           
  Lines        8975     8975           
=======================================
  Hits         6753     6753           
  Misses       2222     2222           

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