[1.4.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.

Backpatch of 9f8a36eb20895d9e542820d5190bfa77ad1b85d9 from stable/1.5.x.

django/contrib/auth/tests/hashers.py

@@ -14,6 +14,10 @@ except ImportError:
 
 try:
     import bcrypt
+    # Django 1.4 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