[1.5.x] Fixed #21248 -- Skipped test_bcrypt if no py-bcrypt found

Pre 1.6 Django worked only with py-bcrypt, not with bcrypt. Skipped
test_bcrypt when using bcrypt to avoid false positives.

django/contrib/auth/tests/hashers.py

@@ -16,6 +16,10 @@ except ImportError:
 
 try:
     import bcrypt
+    # Django 1.5 works only with py-bcrypt, not with bcrypt. py-bcrypt has
+    # '_bcrypt' attribute, bcrypt doesn't.
+    if not hasattr(bcrypt, '_bcrypt'):
+        bcrypt = None
 except ImportError:
     bcrypt = None