FIX: add render_mode attribute

[younik]

This PR addresses https://github.com/mgbellemare/Arcade-Learning-Environment/issues/471 and https://github.com/mgbellemare/Arcade-Learning-Environment/issues/475 by adding the property render_mode to AtariEnv.

Quick test:

import gym
from gym.wrappers import RecordVideo

env = RecordVideo(
    gym.make('ALE/Breakout-v5', render_mode="rgb_array"),
    video_folder='video'
)

env.reset()
for _ in range(100):
    env.step(env.action_space.sample())

env.close()

@JesseFarebro

[JesseFarebro]

Thanks for fixing this! 🙌