Exclude files from .gitignore

[mkniewallner]

Resolves #200.

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

This PR implements the changes suggested in the linked issue with the help of pathspec, with a slight modification: it doesn't empty the default exclude list. Since not all projects use git, I figured that it might be better to keep this default list.

[codecov-commenter]

Codecov Report

Merging #248 (0991b01) into main (01f6b83) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #248   +/-   ##
=====================================
  Coverage   95.6%   95.7%           
=====================================
  Files         32      32           
  Lines        899     919   +20     
  Branches     189     192    +3     
=====================================
+ Hits         860     880   +20     
  Misses        26      26           
  Partials      13      13           

Impacted Files	Coverage Δ
deptry/cli.py	93.4% <100.0%> (+0.1%)	⬆️
deptry/core.py	97.6% <100.0%> (+<0.1%)	⬆️
deptry/python_file_finder.py	100.0% <100.0%> (ø)

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