Bug 1696530 - Run reftests with py3; r=ahal

Run all flavors of reftest with py3. A minor mozharness adjustment
is needed to stop using a Windows 7 work-around needed with py2 only.

Differential Revision: https://phabricator.services.mozilla.com/D111574

taskcluster/ci/test/reftest.yml

@@ -11,6 +11,7 @@ job-defaults:
             android-hw-.*: geckoview-androidTest.apk
             default: null
     variants: ['fission']
+    python-3: true
     tier:
         by-variant:
             fission: 2

testing/mozharness/scripts/desktop_unittest.py

@@ -19,7 +19,6 @@ import copy
 import shutil
 import glob
 import imp
-import platform
 
 from datetime import datetime, timedelta
 
@@ -1198,14 +1197,6 @@ class DesktopUnittest(TestingMixin, MercurialScript, MozbaseMixin, CodeCoverageM
                     # 3) checking to see if the return code is in success_codes
 
                     success_codes = None
-                    if (
-                        suite_category == "reftest"
-                        and "32bit" in platform.architecture()
-                        and platform.system() == "Windows"
-                    ):
-                        # see bug 1120644, 1526777, 1531499
-                        success_codes = [1]
-
                     tbpl_status, log_level, summary = parser.evaluate_parser(
                         return_code, success_codes, summary
                     )