update dependencies, migrate build-backend to hatchling

[alexrashed]

Motivation

We are currently planning to add additional support for hatchling, such that entrypoints can dynamically be detected when hatchling is being used as a build-backend for a project which uses plux.
Similar to pytest, hatchling uses pluggy, and the old pytest version caused a version conflict.
This is why I updated all dependencies, moved from setup.cfg to pure pyproject.toml, and along the way I ended up migrating the whole project to hatchling.
Please let me know if I went a bit too far here, but hatchling seems to be the clear successor of setuptools, so it seemed just natural to implement the migration.

Changes

• Drop support for Python 3.8 (EOL was October 2024).
• Add official support for Python 3.13.
• Migrate from setup.cfg to pure pyproject.toml and remove setup.py.
• Upgrade all dependency pins.
• Migrate the build backend from setuptools to hatchling.