Update env_stocktrading_np.py

[krishdotn1]

I think Finrl use latest version of stable_baselines3 and it requires gymnasium environment which you need to return 5 variables from step function and 2 variables from reset function.

it could generate error who is using latest version of stable-baselines3 and gymnasium.

[zhumingpassional]

@krishdotn1 k I agree with you