Code review comment for ~cjwatson/launchpad:six-dict-iter

I totally agree with iterating over the dict instead of doing dict.iterkeys(), but is it really necessary to use `six.iter{keys,values,items}`?

I mean, under the hood six's methods are basically `return iter(dict.values())` (for values, keys and items), and `keys.values()` works fine both on python2 and python3.

The only downside of using directly `dict.values` on Python2 that I can spot is the overhead of creating a copy list of all values, but that's exactly what six does anyway.