bpo-29957: change LBYL key lookup to dict.setdefault

[selik]

The results was constructed as a defaultdict and we could simply
delete the check if key not in results. However, I think it's safer
to use dict.setdefault as I'm not sure whether the caller expects a
regular dict or defaultdict.

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[benjaminp]

Why switch to setdefault when the defaultdict would work?

[selik]

The caller may expect that looking up a missing key will raise KeyError instead of inserting with an empty list. The defaultdict didn't bother anyone before, presumably, but this might make it just a tad friendlier.

I don't know enough about the context here for how the run function will be used, but given that the docstring says it returns a "dictionary of node matches with fixers as the keys" and doesn't mention that it's a defaultdict, returning exactly a dict may be best.

[selik]

I switched it back to defaultdict. @rhettinger says it'll be "a little more compact, faster, and nice looking" (http://bugs.python.org/msg290993).