Well, all of the buildbot builds are compiled with Python 2, not 3. You’d have to grab the latest code from CVS in order to build with Python 3 support. You have to replace the Python version in thirdparty/win-python-x64 (or win-python for 32-bit systems) simply by removing that directory, running the appropriate Python 3 installer and pointing it at that directory (hit “install for this user only”).
You will need to see this in order to compile latest CVS: