Tut: upgrade to Django1.11 and make sure it also works with Django 1.8

Disable the suppressing of warnings in `pytest.ini`: and fix all deprecation errors.
Fix stuff so deprecation errors are gone
Then update the travis file to also test in Django3.7.
Update Django in the requirements to Django1.11

Bonus points if you can keep the tests green for both Django versions.

Start with pytest.ini, the requirements file, and the Travis configuration. Run the tests with warning suppression disabled, identify the reported deprecations, update the Django requirement and CI matrix as specified, and verify that the suite remains green for the requested Django versions.

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