Add Pettingzoo Bindings

**TLDR**: [Petting Zoo](https://www.pettingzoo.ml/) has become the standard library for getting multi-agent environments & we want to support Petting Zoo's bindings in gym-microrts.

This project https://github.com/vwxyzjn/gym-microrts is an RL environment for RTS game, where lots of units are always spawning and dying. Because of the multi-agent nature of RTS games, gym-microrts should fit with PettingZoo’s interface pretty seamlessly.

We currently need help on the following fronts:
1. Support PettingZoo’s API in gym-microrts. The current API is already similar to PettingZoo’s API (having similar observation space, action space, and support for action masks) but it will be nice to officially adopt PettingZoo.
2. Making SB3 work with pettingzoo. Recently, we had a contribution that made SB3 work with gym-microrts by @kachayev: https://github.com/kachayev/gym-microrts-paper-sb3, it would be nice to have an SB3 demo that works with gym-microrts’s pettingzoo API and ultimately support all pettingzoo environments that have action masks such as Chess or Go.

Setting up an issue to track progress.

@BolunDai0216 suggests he would like to take a stab at this.