Unreachable code when user is_active is False in.

[tzimos]

django-rest-framework-jwt/rest_framework_jwt/serializers.py

Line 53 in 4021e0b

if not user.is_active:

When using the default ModelBackend in django settings activated,
the authenticate method in django.contrib.auth.backends.ModelBackend has the user_can_authenticate method.
In our case user.is_active is False and as a result the whole authenticate method in
this file returns None.
As a result in line 53 there is a check if the user is_active but it is unreachable in this point
and we render the wrong message which is in line 64.

[prairiewest]

Please look at #484. The project is unmaintained, and you should close this issue.