Use Django's six module for StringIO imports.

helpdesk/forms.py

@@ -8,10 +8,7 @@ forms.py - Definitions of newforms-based forms for creating and maintaining
 """
 from django.core.exceptions import ObjectDoesNotExist
 
-try:
-    from StringIO import StringIO
-except ImportError:
-    from io import StringIO
+from django.utils.six import StringIO
 
 from django import forms
 from django.forms import extras

helpdesk/models.py

@@ -1388,7 +1388,7 @@ class CustomField(models.Model):
     )
 
     def _choices_as_array(self):
-        from StringIO import StringIO
+        from django.utils.six import StringIO
         valuebuffer = StringIO(self.list_values)
         choices = [[item.strip(), item.strip()] for item in valuebuffer.readlines()]
         valuebuffer.close()