Bernie Hackett, Anne Herlihy, and Luke Lovett released PyMongo 2.9 last week. "Why," you ask, "was PyMongo 2.9 released after PyMongo 3?"
PyMongo 2.9 is a bridge for PyMongo 2.8 users who want to upgrade applications to PyMongo 3's new API. There are substantial API changes in the 3.0 release, so the PyMongo team created a version that supports nearly the entire APIs for both PyMongo 2 and 3. That's PyMongo 2.9.
Read the Migration Guide for complete instructions. The short version is: upgrade your application to PyMongo 2.9 first. Turn on DeprecationWarnings like:
python -Wd my_application.py
Once you have run your tests and fixed all your uses of deprecated PyMongo 2 APIs, you are most of the way to PyMongo 3 readiness. There are a few API changes this technique can't catch, so read the rest of the migration guide carefully. Once you've done that, upgrade to PyMongo 3 and run your tests once again. Now you've safely reached PyMongo 3, with all the performance and robustness enhancements it offers.