Merge updates back to main branch

[fangchenli]

Last year, I thought this project was abandoned and made some updates on my fork. https://github.com/fangchenli/pyhocon
And it seems like there were some activities recently. I wonder if my fork could get merged back to the main branch.

On my fork:

• Dropped Python 2 support
• CI using GH
• Test all three OS and Python 3.7-3.10
• Use black, isort, and pre-commit for linting
• Some minor cleanup.

All changes are maintenance related.