Installing this package will allow you to utilize the 2020+ WPILib GUI DS Socket from a RobotPy program.
First, install pyfrc. Then run your robot with the 'sim' argument and --ds-socket flag:
# Windows
py -3 ./robot.py sim --ds-socket
# Linux/OSX
python3 robot.py sim --ds-socket
WPILib's documentation for using the simulator can be found at http://docs.wpilib.org/en/latest/docs/software/wpilib-tools/robot-simulation/