Replaced cStringIO.StringIO by io.BytesIO.

Also replaced StringIO.StringIO by BytesIO in some other appropriate
places. StringIO is not available in Python 3.

tests/modeltests/model_forms/tests.py

@@ -1372,19 +1372,6 @@ class OldFormForXTests(TestCase):
         self.assertEqual(instance.image.name, 'foo/test4.png')
         instance.delete()
 
-        # Test image field when cStringIO is not available
-        from django.forms import fields
-        from StringIO import StringIO
-        old_StringIO = fields.StringIO
-        fields.StringIO = StringIO
-        try:
-            f = ImageFileForm(
-                data={'description': u'An image'},
-                files={'image': SimpleUploadedFile('test.png', image_data)})
-            self.assertEqual(f.is_valid(), True)
-        finally:
-            fields.StringIO = old_StringIO
-
     def test_media_on_modelform(self):
         # Similar to a regular Form class you can define custom media to be used on
         # the ModelForm.