WIP attempt fix build to be PEP 517 compatible

[salotz-sitx]

My first attempt at supporting PEP 517 (i.e. pyproject.toml).

This approach uses Cython as a build dependency.

Feel free to dismiss, just wanted to share in case it might be helpful.

It also tried to clean a number of things up in the script that wouldn't be needed if leveraging PEP 517.