[WIP] PAPAG and SB3 benchmark against the same environment component

[nickjalbert]

WIP, some notes:

• I pinned the gym[atari]==0.19.0 requirement because later gym versions moved the Atari ROMs out of the package and into subordinate packages. This complicates selecting a particular class to make into an environment component, so I'm just sidestepping this for now by requiring an earlier gym version.

• I had to copy over some chunks of PAPAG code in example_agents/papag/agent.py and then modify it so that it would play nice with PCS. That code is fairly complicated and I had to jump through some extra hoops to make serialization work as PAPAG expected.

• SB3 is not fully ported yet, but PAPAG can play either Pong:

agentos run agent --entry-point learn --arg-set-file a2c_pong_args.yaml

or CartPole:

agentos run agent --entry-point learn --arg-set-file a2c_cartpole_args.yaml

• Tests are broken because SB3 is not ported yet.

[nickjalbert]

@andyk FYI, here's the work in progress for SB3 and PAPAG running against the same environment (Pong) component. This should be working now, for PAPAG:

agentos run agent --entry-point learn --arg-set-file ppo_pong_args.yaml
agentos run agent --entry-point evaluate --arg-set-file ppo_pong_args.yaml

and for SB3

agentos run sb3_agent --entry-point learn
agentos run sb3_agent --entry-point evaluate

[nickjalbert]

Closing; new WIP branch is nj_ilya_bench3