[1.8.x] Refs #24324 -- Fixed UnicodeDecodeError in template_backends tests

The message for the SuspiciousFileOperation exception needs to
be a unicode string.

Backport of bebc1e53a3 from master

django/utils/_os.py

@@ -1,3 +1,5 @@
+from __future__ import unicode_literals
+
 import os
 import stat
 import sys