This provides two benefits:

• We can finally automate release duties in Evergreen. There is no CPython 3.3 installer from python.org that works on modern macOS.
• Another large reduction in the size of our Evergreen matrix.

CPython 3.3 hasn't had a release (even a security release) since 2014. It officially reaches end of life before PyMongo 3.6 will ship, other popular packages are also about to drop support for it (for example, pip) and there is almost no download activity for it on pypi.