fix(dependency_getter): handle strings for `setuptools` dynamic dependencies

[mkniewallner]

Closes #944.

PR Checklist

• [x] A description of the changes is added to the description of this PR.
• [x] If there is a related issue, make sure it is linked to this PR.
• [x] 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

It might not be obvious from setuptools documentation, but file attribute can either be a string or a list of strings.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.0%. Comparing base (40765df) to head (4cab592). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #945 +/- ## ===================================== Coverage 93.0% 93.0% ===================================== Files 37 37 Lines 991 994 +3 Branches 97 98 +1 ===================================== + Hits 922 925 +3 Misses 55 55 Partials 14 14 ```

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