Drop the use for django.utils.six

[EricFelixLuther]

Django 3.0 dropped the support for the six library. In order to keep up with evolving framework, this should be changed and fixed.

[ngkabra]

I ran into this issue and managed to fix it by installing django-utils-six.

Until the use of django.utils.six is removed from django-dbtemplates maybe django-utils-six should be added as a dependency?

[emilkjer]

Duplicate #107

[hugovk]

django.utils.six was replaced by six in #107, and six and Python 2 dropped in #112.