Remove TODO item for cache expiration
Demote log message when xdg is missing from error to info

Otherwise the log messages clutter up the mutt screen
Support older versions of pyxdg

save_cache_path was added in 0.21
See: https://bugs.freedesktop.org/show_bug.cgi?id=26458

Fall back to ~/.cache and make sure the directory exists
(that's what save_cache_path does)