Fix UnicodeDecodeError

If permission details contains non-ASCII symbols this line produces UnicodeDecodeError, eg:
`task_cloner | Задание на клонирование | Может набирать задания` 
-> 
`UnicodeEncodeError: 'ascii' codec can't encode characters in position 33-39: ordinal not in range(128)`.
You should use `unicode_literals` (see https://code.djangoproject.com/ticket/18269)