Make tests fail on warnings

This is a technique I've rolled out against my own open source projects and [Django REST Framework](encode/django-rest-framework#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 django-import-export#1195, saving everyone some time, effort, and noise.

Author: adamchainz

tox.ini

@@ -8,7 +8,7 @@ envlist =
        {py36,py37,py38}-djangomaster-tablib{dev,stable},
 
 [testenv]
-commands = python {toxinidir}/tests/manage.py test core
+commands = python -W error::DeprecationWarning -W error::PendingDeprecationWarning {toxinidir}/tests/manage.py test core
 deps =
     tablibdev: -egit+https://github.com/jazzband/tablib.git#egg=tablib
     tablibstable: tablib