build: use lighter poetry-core

[branchvincent]

I noticed the project is currently using the full poetry package as its build backend. There's now a preferred, lightweight poetry-core package. What's the benefit? To quote the project's readme:

Prior to the release of version 1.1.0, Poetry was a build as a project management tool that included a PEP 517 build backend. This was inefficient and time consuming in majority cases a PEP 517 build was required... This makes PEP 517 builds extremely fast for Poetry managed packages.

This change is especially helpful for distributors (such as Homebrew) to build from source without requiring all of poetry.