Fix remaining Python 3 compatibility issue

It wasn't until today that I ran `sparse list` on a streamparse project (I've mostly been using the bolt implementation so far) and encountered an issue with trying to index into a `dict_keys` object. That's fixed now.

I also removed an extra preposition in an error message.
Merge pull request #37 from dan-blanchard/patch-1

Fix remaining Python 3 compatibility issue that breaks sparse list.