perf(finder): more efficient Python files retrieval

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Noticed a bunch of performance improvements that could be made to the method that retrieves Python files.
Tested over a few projects with some ignored patterns, and it reduced the duration spent in the method by ~25%.

[codecov-commenter]

Codecov Report

Merging #243 (53ae19d) into main (a2866fe) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #243     +/-   ##
=======================================
- Coverage   95.7%   95.7%   -0.1%     
=======================================
  Files         32      32             
  Lines        915     913      -2     
  Branches     203     201      -2     
=======================================
- Hits         876     874      -2     
  Misses        26      26             
  Partials      13      13             

Impacted Files	Coverage Δ
deptry/python_file_finder.py	100.0% <100.0%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.