Passing the `-3` option to Python 2 switches on DeprecationWarnings about Python 3 incompatibilities: https://docs.python.org/2/using/cmdline.html#cmdoption-3 By using `-3` mode on Travis, it will help prevent regressions in the Python 3 fixes that have already landed. This mode already helped find one real issue that affects us even under Python 2 (bug 1453837).

Commit pushed to master at https://github.com/mozilla/treeherder https://github.com/mozilla/treeherder/commit/023d31885a8c9d421450713a871e6576c85794a8 Bug 1453850 - Enable Python 2's `-3` mode on Travis (#3444) This enables `DeprecationWarnings` for things that Python 2 knows are not compatible with Python 3. The `error` entry in the pytest `filterwarnings` setting ensures these will be surfaced as Exceptions and so result in test failures. See: https://docs.python.org/2/using/cmdline.html#cmdoption-3 The removal of `sorted()` from `test_bug_job_map_api.py` is to fix: `DeprecationWarning: dict inequality comparisons not supported in 3.x`