Switch gym dependency to gymnasium

[giotherobot]

Since gym is deprecated in favor of gymnasium (see https://github.com/openai/gym?tab=readme-ov-file#important-notice) here I propose the switch, allowing both installation as alternative to keep backwards compatibilty.

This allows for use of python>=3.10, that is incompatible with gym.

Possible fix for #421.

[lebrice]

Just a note: I think the Gym wrappers here use the gym step format, not the gymnasium step format with terminated and truncated. Perhaps these would also need to be updated, as well as the pytorch training notebook that used those wrappers.

[btaba]

Hi @lebrice can you add gymnasium to the requirements, so that the builds pass? Thank you!

[giotherobot]

The idea was to keep both the gym and gymnasium dependencies and let the user install the preferred one, I guess this needs modification of the CI to install one of the two.

Or if you prefer I can drop the gym dependency and just keep the gymnasium one.

[btaba]

Ah ok, sorry I was misreading this. I think the right thing to do is what @lebrice is saying. Switch to gymnasium altogether and hash out differences in the API calls in brax. You can ignore brax/v1 entirely since it's deprecated. How does that sound?

[giotherobot]

I can work on that, should be easier than what I was trying to do here!