Update integration tests to support Django 3 and drop Django 1 and Python 2.7

[B-T-D]

I've created a local version of this library that passes all the included unit tests for Python 3.8, Django 3.0.2, and the long-term-support Django 2.2. But it's not compatible with Python 2 or Django 1. So if I open a pull request now, it's not going to pass the integration tests as currently configured.

I propose dropping the relevant TOX_ENVs from .travis.yml and adding new ones for Python 3.7 and 3.8, and for Django 3.0.

• Python 2.7 reached end-of-life status on January 1, 2020 (https://devguide.python.org/devcycle/#devcycle.

• Django 1.11 long term support is expected to end in April 2020 (https://www.djangoproject.com/download/#supported-versions.

I think some people might now be encountering problems integrating this into Django 3.0 projects. So I think the benefit of updating for Django 3 compatibility outweighs any costs of ending Django 1 and Python 2.7 support.

[pgiki]

I managed to get to work with Python 3.6 and Django 3.1. To do so I replace line ""from django.utils.encoding import python_2_unicode_compatible""" with ""from six import python_2_unicode_compatible""" in the entire project. I use sublime text so just did find and replace it in all folders. You may use any other find and replace methods of your choice.