Fix gym->gymnasium import in SQIL implementation.

src/imitation/algorithms/sqil.py

@@ -8,7 +8,7 @@
 
 import numpy as np
 import torch as th
-from gym import spaces
+from gymnasium import spaces
 from stable_baselines3 import dqn
 from stable_baselines3.common import buffers, policies, type_aliases, vec_env
 from stable_baselines3.dqn.policies import DQNPolicy