In the p3d environment, there isn’t necessarily a “python” executable on the PATH. There’s no reason to assume that the user running the p3d file even has Python installed at all (and a full installation of Python isn’t part of the p3d download).
So while the subprocess module is technically available, it will be difficult to use it to launch a subordinate Python process. I think you will need to find another way to launch your game.
David