fix syntax error in python2 code

we can't use {**dict1, **dict2} in python2 code.
see report https://travis-ci.com/bmsdave/v8/jobs/180113432

Bug: v8:935200
Change-Id: I2f2a1283583432d6d0167436dd7d4205455049cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1484303
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60039}

tools/locs.py

@@ -211,7 +211,8 @@ def SetupReportGroups():
                            "third_party": '\\.\\./\\.\\./third_party',
                            "gen": 'gen'}
 
-  report_groups = {**default_report_groups, **dict(ARGS['group'])}
+  report_groups = default_report_groups.copy()
+  report_groups.update(dict(ARGS['group']))
 
   if ARGS['only']:
     for only_arg in ARGS['only']: