build: migrate from setuptools to poetry (use pyproject.toml for build)

[rgon]

This PR migrates the build system to poetry, greatly simplifying the build, and updates the documentation to reflect the change. Installs now require a no-hastle 'pip install .'. Ensures proper version pinning on development, as well.

closes #154 #141 #47 and also, since PyQT5 apparently now has arm builds and versions are now pinned and on their own venv, #119 #47

Also is a step forward towards closing #15. To close it, only poetry publish is needed, although publishing it with a Github Action will probably be wiser to ensure no more work is required.

Implements part of #149, and is a step towards easily building distributable binaries on GHA in the future.

@bordaigorl LMK if you have any issues running it following the new README (if it's clear enough).

• Gonzalo