Modernize the CSRF migration helper.

Thanks Lennart Regebro for spotting that code.

extras/csrf_migration_helper.py

@@ -281,12 +281,10 @@ def search_python_list(python_code, template_names):
     Returns a list of tuples, each one being:
      (filename, line number)
     """
-    retval = []
+    retval = set()
     for tn in template_names:
-        retval.extend(search_python(python_code, tn))
-    retval = list(set(retval))
-    retval.sort()
-    return retval
+        retval.update(search_python(python_code, tn))
+    return sorted(retval)
 
 def search_python(python_code, template_name):
     """