Make tests fail on warnings (#1197)
Make tests fail on warnings (#1197)

This is a technique I've rolled out against my own open source projects and [Django REST Framework](https://github.com/encode/django-rest-framework/pull/7586).

By failing tests as soon as new deprecation warnings are created in the library, they aren't passed on to users who then have to report them back in issues like #1195, saving everyone some time, effort, and noise.
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