Bug 1689837 - run compiled tests in python3 r=rjl

Differential Revision: https://phabricator.services.mozilla.com/D103594

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/compiled.yml

@@ -18,6 +18,7 @@ job-defaults:
                 windows.*:
                     - unittests/win_unittest.py
                     - unittests/thunderbird_extra.py
+    python-3: true
 
 gtest:
     description: "GTests run"