Fixed comparison operator according to PEP8.

adminsortable/templatetags/django_template_additions.py

@@ -23,7 +23,7 @@ class DynamicRegroupNode(template.Node):
 
     def render(self, context):
         obj_list = self.target.resolve(context, True)
-        if obj_list == None:
+        if obj_list is None:
             # target variable wasn't found in context; fail silently.
             context[self.var_name] = []
             return ''