Fixed PPO ImportError and removed requests==2.28.0

[KelvinYang0320]

1. The agents folder has been deprecated in favor of algorithms on ray 2.x version. To be backward compatible, we can use try ... except ... to import ppo.

   • https://discuss.ray.io/t/ray-rllib-agents-ppo-missing/9904

2. Removed requests==2.28.0 It seems that requests is already listed in the requirements.txt of ray, so I think we can remove it.

   • https://github.com/ray-project/ray/blob/master/release/requirements.txt
   • https://github.com/aidudezzz/deepworlds/pull/94