[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.