I spent a great deal of time banging my head against the example here:
https://api.mongodb.com/python/current/examples/custom_type.html

It turns out this has been deprecated without replacement, and doesn't work for any of the new methods (insert_one, etc) without mention anywhere in the documentation.

I really think the documentation should make note of this. The depreciation notice should mention it too. I'm not the first person to be confused by this:
http://stackoverflow.com/questions/33289643/pymongo-son-manipulator-transform-not-working-mongodb