Bug 1696531: use python3 for xpcshell tests, r=bhearsum

Depends on D111647

Differential Revision: https://phabricator.services.mozilla.com/D109728

taskcluster/ci/test/xpcshell.yml

@@ -12,6 +12,7 @@ job-defaults:
         by-test-platform:
             android-em-7.*: geckoview-androidTest.apk
             default: null
+    python-3: true
     mozharness:
         script:
             by-test-platform: