!223 broke HTML summary generation

Due to leftover six.iteritems() in templates/testrun_info.mako:

> ./piglit summary --overwrite html summary/gpu results/gpu.2020021{3,4}-baseline                                                                                                     ~/src/piglit-git/piglit
Traceback (most recent call last):
  File "./piglit", line 170, in <module>
    main()
  File "./piglit", line 166, in main
    sys.exit(runner(args))
  File "/home/daenzer/src/piglit-git/piglit/framework/exceptions.py", line 49, in _inner
    func(*args, **kwargs)
  File "/home/daenzer/src/piglit-git/piglit/framework/programs/summary.py", line 115, in html
    summary.html(args.resultsFiles, args.summaryDir, args.exclude_details)
  File "/home/daenzer/src/piglit-git/piglit/framework/summary/html_.py", line 188, in html
    _make_testrun_info(results, destination, exclude)
  File "/home/daenzer/src/piglit-git/piglit/framework/summary/html_.py", line 109, in _make_testrun_info
    info=each.info))
  File "/usr/lib/python3/dist-packages/mako/template.py", line 476, in render
    return runtime._render(self, self.callable_, args, data)
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 883, in _render
    **_kwargs_for_callable(callable_, data)
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 920, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 947, in _exec_template
    callable_(context, *args, **kwargs)
  File "/tmp/daenzer/3198714166/python-3.7.6/mako-1.1.0/summary/html/html-summary/testrun_info.mako.py", line 36, in render_body
    for key, value in sorted(six.iteritems(totals), key=lambda i: (i[1], i[0]), reverse=True):
AttributeError: 'Undefined' object has no attribute 'iteritems'

(This should probably be tested in the CI pipeline :)

/cc @dbaker