core (constants): better management of xdg import

src/core/constants.py

@@ -17,9 +17,11 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from xdg import BaseDirectory
-if BaseDirectory is not None:
+try:
+    from xdg import BaseDirectory
     from os.path import expanduser, realpath
+except ImportError:
+    BaseDirectory = None
 
 
 class Const(object):