support isolated build via pyproject

[tlambert03]

Hello,

I know this project is largely in maintenance mode now, but I'm helping someone who very much likes using it and I'm trying to help make their project easier to install. One hurdle to that is the fact that you can't pip install lap into a fresh environment without first installing numpy/cython. PEPs 517/518 provide a solution for that, and have been implemented here by adding a pyproject.toml file.

I think these are the minimum changes needed to support pip install lap without making any assumptions about the existing environment. I hope you'll consider this small change and push an updated sdist to pypi (i updated the RELEASING.md guide accordingly)

[tlambert03]

sorry, I just also noticed #52 @cshimmin ... that would also be great! :)

edit ... and #46 :joy:

hopefully @gatagat or anyone else with maintenance permissions here will have time to review a solution here