chore(ci): Apply Repo-Review suggestions [mypy]

[DimitriPapadopoulos]

Apply all suggestions related to mypy:

https://learn.scientific-python.org/development/guides/repo-review/?repo=bids-standard%2Fpython-validator&ref=HEAD&refType=branch

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.89%. Comparing base (6fe32d1) to head (61f8557).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bids_validator/types/files.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #60   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files          13       13           
  Lines         846      846           
  Branches      124      124           
=======================================
  Hits          769      769           
  Misses         47       47           
  Partials       30       30           

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