[Bug Report] Spec.to_json() of SimpleCrossing fails

[younik]

Describe the bug The spec dict of SimpleCrossing environments is not serializable due to obstacle_type.

Code example

import gymnasium as gym
import minigrid

gym.make("MiniGrid-SimpleCrossingS11N5-v0").spec.to_json()

System Info Minigrid installed via pip install minigrid. I am using OSX, with Python 3.11

Checklist

• [x] I have checked that there is no similar issue in the repo (required)