Gym-Gymnasium compatibility converter

[RedTachyon]

Based on many external discussions, this implements a way to use gym environments inside gymnasium.

They can be created in two ways -- either pass it by name with optional arguments to create an env from the gym registry:

import gymnasium as gym
gym.make("GymV26Environment-v0", env_id="CartPole-v0", make_kwargs={...})

or use an arbitrary instantiated gym env

import gymnasium
import gym
env = gym.make(...)
gymnasium.make("GymV26Environment-v0", env=env)

At the moment it only supports the modern API used in gym 0.26, in the future we might add support for the old API, though at that point it might enter a dependency hell. We'll see.

[pseudo-rnd-thoughts]

A couple of initial comments

[pseudo-rnd-thoughts]

Code is all good, could you update the PR top comment from "WiP" to be more descriptive and provide a short description of the PR rational over the prior ideas

[pseudo-rnd-thoughts]

Do we want a __get_attr__ function to get attributes from the gym environment

[RedTachyon]

Do we want a __get_attr__ function to get attributes from the gym environment

I'd rather not do that, I'm already slightly uneasy with the way getattr is handled in wrappers. If users need to access something from the backend environment, they can just do env.gym_env.something and it's clear where it comes from.