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