bpo-29957: change LBYL key lookup to dict.setdefault (#938)

* change LBYL key lookup to dict.setdefault

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.

* add name to the acknowledgements file

* use defaultdict to make the key-lookup cleaner