Deveria ser :
from pymongo.objectid import ObjectId
thing = db.things.find_one({'_id': ObjectId('4ea113d6b684853c8e000001') })
EDIT:A importação atual é:
from bson.objectid import ObjectId
from pymongo.objectid import ObjectId
thing = db.things.find_one({'_id': ObjectId('4ea113d6b684853c8e000001') })
from bson.objectid import ObjectId