Replace appdirs with platformdirs

[hugovk]

platformdirs is a better-maintained fork.
ActiveState/appdirs#79

[codecov]

Codecov Report

Merging #23 (6d28d6a) into main (4d608c6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   97.29%   97.29%           
=======================================
  Files           6        6           
  Lines         296      296           
=======================================
  Hits          288      288           
  Misses          8        8           

Flag	Coverage Δ
GHA_Ubuntu	97.29% <100.00%> (ø)
GHA_Windows	97.29% <100.00%> (ø)
GHA_macOS	97.29% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/norwegianblue/__init__.py	97.22% <100.00%> (ø)

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