Replace the word obsolete with unused

[fpgmaas]

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

• Replaces all occurences of the word 'obsolete' with the word 'unused'.
• For now, the skip_obsolete and ignore_obsolete arguments will still remain functional. However, if both the new and the to-be-deprecated arguments are passed, an error is raised. See deprecate_obsolete.py
• Added various unit tests. here and here

[codecov]

Codecov Report

Merging #373 (d742709) into main (18dad91) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #373   +/-   ##
=====================================
  Coverage   96.0%   96.1%           
=====================================
  Files         37      38    +1     
  Lines       1042    1065   +23     
  Branches     207     213    +6     
=====================================
+ Hits        1001    1024   +23     
  Misses        25      25           
  Partials      16      16           

Impacted Files	Coverage Δ
deptry/cli.py	92.0% <100.0%> (+0.3%)	⬆️
deptry/core.py	98.0% <100.0%> (ø)
deptry/deprecate_obsolete.py	100.0% <100.0%> (ø)
deptry/issues_finder/unused.py	93.5% <100.0%> (ø)
deptry/violations/__init__.py	100.0% <100.0%> (ø)
deptry/violations/unused.py	100.0% <100.0%> (ø)

[mkniewallner]

Played with the PR a bit locally, it works as expected 🎉