fix readthedocs build error

Trinkle23897 · Trinkle23897 · commit c8be85b2407b · 2023-02-03T14:55:53.000-08:00
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -7,3 +7,4 @@ tensorboard
 torch
 tqdm
 protobuf~=3.19.0
+pettingzoo
