Fix path building for config dir and data dir

The paths are now correctly built in the case where $XDG_DATA_HOME and
$XDG_CONFIG_HOME are not empty: the "weboob/" suffix was in the fallback
argument of the os.get.environ.

Signed-off-by: Olivier Schwander <olivier.schwander@ens-lyon.org>
Signed-off-by: Romain Bignon <romain@symlink.me>