This sounds like a reasonable idea to me, and we can point folks to the original appdirs if they need a version-independent alternative.
Depending on the amount of code involved, we could potentially consider providing this as an API *in* pathlib, rather than needing an entire new module for the standard library version of it.
If this get into python, it would be nice to have a `python -m <module> <appname>` that return to user the config dirs. One of the most challenging issues we have with user is “where is my config/cache/...” and it’s always hard start the response by “It depends of...”. The “run this command to know” works better.