[1.8.x] Refs #24324 -- Fixed UnicodeDecodeError in template_backends tests
[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 bebc1e53a3ab059849e5c4e5a55b2f5e68b67169 from master

h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,h4r5h1t/django-hauthy,h4r5h1t/django-hauthy,pquentin/django,pquentin/django