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 95.92%. Comparing base (24109a1) to head (78d8270).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2769   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files         377      377           
  Lines       53535    53535           
=======================================
  Hits        51356    51356           
  Misses       2179     2179           

Flag	Coverage Δ
behaviourtests	4.10% <66.66%> (ø)
unittests	96.02% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 8534439987

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.008%

---

Totals
Change from base Build 8526036089:	0.0%
Covered Lines:	51228
Relevant Lines:	53358

---

💛 - Coveralls

[mraspaud]

LGTM, thanks a lot for bringing this up!