Silence the MiddlewareMixin deprecation warning explicitly (#2416)

Author: matthiask

tox.ini

@@ -39,7 +39,9 @@ setenv =
         error::PendingDeprecationWarning,\
         ignore:The RedirectsPanel is deprecated:DeprecationWarning,\
         # This can be removed when we're fully to 6.0
-        ignore:The 'partial'and 'partialdef' template tags are now part of Django core:DeprecationWarning
+        ignore:The 'partial'and 'partialdef' template tags are now part of Django core:DeprecationWarning,\
+        # django-csp imports MiddlewareMixin from the deprecated location; not ours to fix
+        ignore:Importing MiddlewareMixin from django.utils.deprecation is deprecated
     DB_NAME = {env:DB_NAME:debug_toolbar}
     DB_USER = {env:DB_USER:debug_toolbar}
     DB_HOST = {env:DB_HOST:localhost}