Remove argparse.FileType

[kylebittinger]

The use of argparse.FileType now results in a pending deprecation warning. Here, I removed all calls to this class and refactored the code to open the files directly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.07%. Comparing base (18c9d58) to head (87dcec2).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   95.04%   95.07%   +0.03%     
==========================================
  Files          11       11              
  Lines         565      569       +4     
==========================================
+ Hits          537      541       +4     
  Misses         28       28              

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