Does not work with the latest mujoco 3.1.1

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.