fixed my error

replaced django.conf.urls with django.core.urlresolvers.

sitetree/admin.py

@@ -3,7 +3,7 @@
 try:
     from django.urls import get_urlconf, get_resolver
 except ImportError:
-    from django.conf.urls import get_urlconf, get_resolver
+    from django.core.urlresolvers import get_urlconf, get_resolver
 from django.utils.translation import ugettext_lazy as _
 from django.utils import six
 from django.http import HttpResponseRedirect