tox: Don't use exceptions - just warn

It seems Django uses deprecated items in Python 3.x, meaning we can't
use this.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 9db5cd0 ("tox: Turn deprecation notices into warnings")

Author: stephenfin

tox.ini

@@ -26,7 +26,7 @@ passenv =
     PW_TEST_DB_PORT DISPLAY SELENIUM_BROWSER PW_SKIP_BROWSER_TESTS
     DISPLAY HOME XAUTHORITY
 commands =
-    python -Werror {toxinidir}/manage.py test --noinput \
+    python -Wonce {toxinidir}/manage.py test --noinput \
         --liveserver=localhost:9000-9200 '{posargs:patchwork}'
 
 [testenv:bashate]