Does not work with the latest mujoco 3.1.1

[ernestum]

When trying to load the swimmer environment, I get a

ValueError: XML Error: Schema violation: unrecognized attribute: 'collision'

error when mujoco 3.1.1 is installed. Downgrading to mujoco==2.3.7 fixes the issue.

-> The dependencies in setup.py are not pinned or constrained at all. Maybe adding a mujoco=~2 will fix this.

[twobox]

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