feat(python_file_finder): improve ignore handling

[mkniewallner]

Closes #882.

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!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

Currently, if a .gitignore file is hanging around in a parent directory that is outside a git repository, deptry will read it. Thinking this through, I believe we should simply avoid reading any .gitignore altogether if not in a git repository, by not disabling require_git. This is in line with what many tools do in the Python ecosystem, one such example being Ruff.

The positive side-effect of doing that is that when in a git repository, ignore crate will only read parents directories to find gitignore files up to when it finds a .git directory, fixing the use case reported in the issue. Newly added tests demonstrate that this fixes the issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.6%. Comparing base (d9a1ba1) to head (ca9e39b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #908   +/-   ##
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  Coverage   92.6%   92.6%           
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  Files         37      37           
  Lines       1006    1006           
  Branches      99      99           
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  Hits         932     932           
  Misses        59      59           
  Partials      15      15           

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[mkniewallner]

https://docs.rs/ignore/0.4.23/ignore/struct.WalkBuilder.html#method.standard_filters

All the options that standard_filters set are enabled by default anyway, so this does not change the current behaviour when not overriding exclusion list. But when overriding it, this allows disabling everything related to ignore (because there actually is more than .gitignore at play).