Replace casefold with lower

[moi15moi]

This was clearly an error.
I don't know why i used casefold everywhere instead of lower.
casefold doesn't behave as we want and also, it doesn't properly emulate vsfilter.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (3d7c718) to head (b48b06b).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files          35       35           
  Lines        1640     1640           
=======================================
  Hits         1551     1551           
  Misses         89       89           

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