truncate error description in admin

Also removes old conditional imports

django_q/tests/test_admin.py

@@ -1,7 +1,4 @@
-try:
-    from django.urls import reverse
-except ImportError: # Django < 1.10
-    from django.core.urlresolvers import reverse
+from django.urls import reverse
 from django.utils import timezone
 
 import pytest