message.py is not using _cmessage (the C extension message module) to package messages for the server. This seems to have been broken when the BSON module was created with these commits:

https://github.com/mongodb/mongo-python-driver/commit/9c182809f3e7ce3b93da99a825d0ad73d07689d9
https://github.com/mongodb/mongo-python-driver/commit/789296c6efc636deb8adf918e534cdad53179256

Fixing the import problem is straightforward but it seems that there are some problems in _cmessagemodule.c likely caused by another commit from the same time period:

https://github.com/mongodb/mongo-python-driver/commit/56699b9ff2279d8d50311b01630f015a8d53eb8f