My guess would be that this is not possible to control within PsychoPy/Python, or any other program. We get the position of the mouse from the operating system, after it applies whatever position/velocity/acceleration algorithm it is set to use. I don’t think it is possible to somehow access a “raw” mouse position signal, before it gets processed by the OS.
It might be possible to achieve what you want by directly making changes via the operating system itself, however, e.g. this for Windows 10:
or this for a Mac:
Neither of those have been tested. Beware that what the OS does to determine the mouse position signal remains a bit of a black box from a user perspective – you probably shouldn’t rely on it being an exact one-to-one mapping between mouse position and resulting pointer movement. And mice inherently track motion, not position – e.g. at any point, the user can lift the mouse off the desk and put it down somewhere else, and the OS will be none the wiser. Disabling mouse acceleration will likely make this behaviour more common, as it is more likely the mouse can get into an uncomfortable location, requiring it to need to be picked up and put down again.