Replace appdirs with platformdirs

[jwodder]

appdirs is deprecated, and platformdirs is its replacement.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9335182) 89.04% compared to head (0e58464) 89.03%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1345      +/-   ##
==========================================
- Coverage   89.04%   89.03%   -0.01%     
==========================================
  Files          76       76              
  Lines       10243    10243              
==========================================
- Hits         9121     9120       -1     
- Misses       1122     1123       +1     

Flag	Coverage Δ
unittests	89.03% <50.00%> (-0.01%)	⬇️

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Files	Coverage Δ
dandi/cli/command.py	46.91% <50.00%> (ø)

... and 1 file with indirect coverage changes

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[github-actions]

🚀 PR was released in 0.58.1 🚀