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/invalid_models/tests.py

@@ -1,6 +1,6 @@
 import copy
 import sys
-from cStringIO import StringIO
+from io import BytesIO
 
 from django.core.management.validation import get_validation_errors
 from django.db.models.loading import cache, load_app
@@ -16,7 +16,7 @@ class InvalidModelTestCase(unittest.TestCase):
         # coloring attached (makes matching the results easier). We restore
         # sys.stderr afterwards.
         self.old_stdout = sys.stdout
-        self.stdout = StringIO()
+        self.stdout = BytesIO()
         sys.stdout = self.stdout
 
         # This test adds dummy applications to the app cache. These