[py3] Used compatible imports of StringIO.

tests/regressiontests/file_uploads/tests.py

@@ -7,12 +7,12 @@ import hashlib
 import json
 import os
 import shutil
-from StringIO import StringIO
 
 from django.core.files import temp as tempfile
 from django.core.files.uploadedfile import SimpleUploadedFile
 from django.http.multipartparser import MultiPartParser
 from django.test import TestCase, client
+from django.utils.six import StringIO
 from django.utils import unittest
 
 from . import uploadhandler