Consider making appdirs an optional dependency

[djmattyg007]

I'd wager there's a large number of users of this library who will never use the appdirs-based FS drivers. I feel it would be better served as an optional dependency, or even to spin them out into a separate package.

[althonos]

I'd be in favor of keeping it because appdirs (or platformdirs, now that we are using it instead, see #489 ) are simple pure-Python dependencies that don't pull any additional dependencies themselves. But since it's our only external dependency it would make sense to remove it when making a Pyfilesystem3.