Publish to pypi

[tanliyon]

Description

This PR adds the files needed to properly publish to PyPI while maintaining all the functionalities.

Specifically:

• agents and examples needs to be within the gym_xiangqi module for it to be found.
• __init__.py files are needed in every directory.
• The non python files needs to be specified in MANIFEST.in.

Our project has also been published to PyPI! Here's the link: https://pypi.org/project/gym-xiangqi/1.1.0/.

It is at version 0.2.0, and versions 1.0.X has bugs from my experimenting.

For v0.2.0, the image on PyPI is not rendered properly, but it has now been fixed and will be fine in the next release. Here's the example on test-pypi: https://test.pypi.org/project/gym-xiangqi/0.2.0/

Type of change

• [] Bug fix (non-breaking changes which fixes an issue)
• [X] New feature (non-breaking changes which adds certain functionality)
• [] Documentation update (updating documentations)
• [] Breaking change (fix that breaks existing functionality)

How has this been tested?

1. Create a new virtual environment.
2. Run pip install gym-xiangqi==1.1.0.
3. Try importing gym_xiangqi.
4. Run agent-v-agent.

[dooki114]

LGTM!