A popular library that has unfortunately not transitioned to Python 3 yet is rospy (which is part of the larger ROS ecosystem for robotics). It is the last framework that prevents me to fully embrace Python 3 for my everyday work. I sincerely hope the robotics community will eventually port rospy (and other ROS-related libraries) to Python 3.
I am, however, very happy to see that ROS2 (the next iteration of ROS) uses Python 3 by default.
I'm in the same boat. It bothers me that my only option will be to rewrite in Python 3. Unless they provide a very clear migration path, I'm far more likely to just remove ROS entirely, unfortunately.
I am, however, very happy to see that ROS2 (the next iteration of ROS) uses Python 3 by default.