Replace the unmaintained dependency appdirs with platformdirs

[avalentino]

• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (c698e31) to head (832fafb).
Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2769      +/-   ##
==========================================
- Coverage   95.93%   95.91%   -0.03%     
==========================================
  Files         366      366              
  Lines       53528    53509      -19     
==========================================
- Hits        51351    51321      -30     
- Misses       2177     2188      +11     

Flag	Coverage Δ
behaviourtests	4.04% <66.66%> (-0.01%)	⬇️
unittests	96.00% <100.00%> (-0.03%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 9368819369

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on feature/platformdirs at 96.031%

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Totals
Change from base Build 9368656053:	96.0%
Covered Lines:	51561
Relevant Lines:	53692

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💛 - Coveralls

[mraspaud]

LGTM, thanks a lot for bringing this up!