The appdirs library has been abandoned for several years now, and a
fork named platformdirs is the recommended alternative. It's almost
a drop-in replacement, except the module and convenience class name
have changed. Adjust the internal variable name we're using for the
expansion to match as well, for all those consistent hobgoblins.
Add it with a >=3 lower bound because that introduces a breaking
change of the default configuration location for macOS users.
Include a release note about that too.
Depends-On: https://review.opendev.org/886642
Change-Id: I4c2b60ca0da29da1f605b4c0b5e77f8e1071a19c
b34de32a54