Things to do to run the Modified MuJoCo Environment:
1. Go to *venv/lib/python3.7/site-packages/gym/envs/mujoco, here substitute *venv with your virtual environment address, and switch to the correct python version that you use in your environment (I use python3.7). 
2. Copy half_cheetah_v3_test.py file to the current directory
3. cd ..
4. Open the __init__.py file, and add the following to the file:
register(
    id='HalfCheetahTest-v3',
    entry_point='gym.envs.mujoco.half_cheetah_v3_test:HalfCheetahTestEnv',
    max_episode_steps=1000,
    reward_threshold=4800.0,
)
5. Now after you import gym, you should be able to do env = gym.make('HalfCheetahTest-v3')
