Fix readthedocs build failure (#803)

Author: Trinkle23897

tianshou/env/utils.py

@@ -4,7 +4,10 @@
 import gymnasium
 import numpy as np
 
-from tianshou.env.pettingzoo_env import PettingZooEnv
+try:
+    from tianshou.env.pettingzoo_env import PettingZooEnv
+except ImportError:
+    PettingZooEnv = None  # type: ignore
 
 if TYPE_CHECKING:
     import gym