The third-party "bson" package in PyPI is unmaintained and unrelated to PyMongo.
Unfortunately, since "pip install pymongo" also installs a top-level "bson" package, users see example code like:
...and assume they should install bson and pymongo from PyPI. Depending on the Python packaging configuration of their system this can lead to a variety of surprising and unintelligible errors. Sometimes immediately, sometimes only after a PyMongo upgrade.
The PyMongo docs should warn against installing "bson" from PyPI, in "installation.rst" and perhaps elsewhere.