Replaced use of TestCase.fail() with assertRaises().

Also removed try/except/fail antipattern that hides exceptions.

tests/sessions_tests/tests.py

@@ -198,13 +198,7 @@ class SessionTestsMixin(object):
         # removed the key) results in a new key being generated.
         try:
             session = self.backend('1')
-            try:
-                session.save()
-            except AttributeError:
-                self.fail(
-                    "The session object did not save properly. "
-                    "Middleware may be saving cache items without namespaces."
-                )
+            session.save()
             self.assertNotEqual(session.session_key, '1')
             self.assertIsNone(session.get('cat'))
             session.delete()