add license in setup.py for pip-license parser

as django-cron is using MIT license 
can add similar like https://github.com/jazzband/django-formtools/blob/master/setup.py
example :
<img width="673" alt="image" src="https://github.com/Tivix/django-cron/assets/82965857/c2c925c6-b8d0-43e9-80c6-20e0a9e86865">
