Use the latest unittest-mixins

Author: nedbat

tox.ini

@@ -21,7 +21,7 @@ envlist =
 
 [testenv]
 deps =
-    unittest-mixins==1.0
+    unittest-mixins==1.1
     django14: Django >=1.4, <1.5
     django15: Django >=1.5, <1.6
     django16: Django >=1.6, <1.7