Deprecation Warning in Django 3.0

[ethanjahn]

Hi Team,

Thanks for the project, it works really well when working with Auth0!

I recently upgraded to django 3.0 which is deprecating django.utils.encoding.smart_text() in favor of django.utils.encoding.smart_str()

I think think smart_str() works as a drop-in replacement for the smart_text() call in line 59 of authentication.py. The only difference between the two is that smart_str() keeps lazy objects as lazy objects, and smart_text() resolves lazy objects to strings.

Happy to submit a PR if you agree that this would not have any unexpected consequences.

Thanks!