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.