FIX: Get rid of basestring (not python3 compatible).

dav_base/tests/test_apps.py

@@ -1,4 +1,5 @@
 from django.apps import apps
+from six import string_types
 
 from .generic import AppSetting, AppsTestCase
 
@@ -7,7 +8,7 @@ class TestCase(AppsTestCase):
     app_config = apps.get_app_config('dav_base')
 
     settings = (
-        AppSetting('email_sender', basestring),
-        AppSetting('email_base_url', basestring),
-        AppSetting('email_subject_prefix', basestring),
+        AppSetting('email_sender', string_types),
+        AppSetting('email_base_url', string_types),
+        AppSetting('email_subject_prefix', string_types),
     )