Bug 1559975 - Fix py3 lint issues in testing/firefox-ui and enable py3 linter r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D35244

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

testing/firefox-ui/harness/firefox_ui_harness/runners/update.py

@@ -105,4 +105,4 @@ class UpdateTestRunner(FirefoxUITestRunner):
         # If exceptions happened, re-throw the last one
         if self.exc_info:
             ex_type, exception, tb = self.exc_info
-            raise ex_type, exception, tb
+            raise ex_type(exception).with_traceback(tb)

tools/lint/py3.yml

@@ -25,7 +25,6 @@ py3:
         - security/manager/ssl
         - servo
         - testing/awsy
-        - testing/firefox-ui/harness/firefox_ui_harness/runners/update.py
         - testing/mochitest
         - testing/mozharness
         - testing/raptor