I've ran into something similar, for me the problem was this line:
from pymongo.objectid import ObjectId
The call seems to work in versions of PyMongo < 2.2
In pymongo 2.2 the call to import objectid is:
from bson.objectid import ObjectId
Looks like they're already aware (see this pull request), but no one's created a pull request against the develop branch yet. You could try fixing that and submit a new pull request.