Add PEP 621 metadata and replace setuptools with a PEP 517 compatible build system

[shenanigansd]

This project does not currently have a pyproject.toml file. Let's go ahead and create one and add project metadata as specified in PEP 621.

Also, now that PEP 517 allows you to specify a specific build system, setuptools is removing their own build system.
You can see the deprecation warning in AppVeyor builds here: https://ci.appveyor.com/project/pglet/pglet-python/builds/42088148#L339
We will need to switch to a PEP 517 compatible build system.