For making an assistive device, does iOS 13 enable control of a cursor using code?
You can use a Bluetooth mouse with iOS 13 by opening Settings > Accessibility > Switch Control > Switches, and then from there add the Bluetooth device. This allows you to control a cursor on screen.

You can also do on-screen navigation using other third party hardware in form of "ordinary" Switch Control equipment. In addition it is possible to use the camera to control navigation - moving your head to the left or to the right choosing diretion.

You can use Python on seperate hardware to emulate a Bluetooth mouse - this would make it possible to control the cursor from Python.

All the above have to do with system wide cursor control. There's also the option of developing your own app. If you do this, you can control the cursor within the app any way you like - this could for example be over the network controlled by Python. This is essentially the way that for example Citrix managed to have a controllable cursor on pre-iOS 13.