Migrate from github3.py to ghapi

This app currently uses `github3.py` for API interactions with GitHub. Since GitHub officially released [`ghapi`](https://github.blog/2020-12-18-learn-about-ghapi-a-new-third-party-python-client-for-the-github-api/#ghapi), it will be less maintenance and fewer missing features to use this official library in the future


https://ghapi.fast.ai/#How-to-use---Python