Django LTS upgrades

[acdha]

Currently the tests will fail on Django 2.2 because they're not using the updated include syntax:

 File "/Users/cadams/Projects/django-tabular-export/tests/urls.py", line 7, in <module>
    urlpatterns = [url(r"^admin/", include(admin.site.urls))]
  File "/Users/cadams/Projects/django-tabular-export/.tox/py37/lib/python3.7/site-packages/django/urls/conf.py", line 27, in include
    'provide the namespace argument to include() instead.' % len(arg)
django.core.exceptions.ImproperlyConfigured: Passing a 3-tuple to include() is not supported. Pass a 2-tuple containing the list of patterns and app_name, and provide the namespace argument to include() instead.