Build system improvements

[friday]

Description

This was among the first thing I started working on when making the v6 branch originally, so I rewrote setup.py to work on newer python versions and using setup.cfg, but since then the community decided to go with PEP-517 and pyproject.toml instead.

https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html
https://drivendata.co/blog/python-packaging-2023

Sub tasks:

• Move refactor: static namespace module #1313
• Refactor to avoid the hack to overwrite init.py in setup.py (done in chore: migrate scripts to makefile #1338)
• Remove every direct call to ./setup.py or python(3) setup.py
• Migrate from setup.cfg to pyproject.toml with setuptools (the most straight forward solution since it's the same as now, but requires setuptools 61.0.0, which is not available in our Docker image, ubuntu 20.04/focal uses setuptools 45.2.0) build: migrate setuptools setup.cfg to pyproject.toml (PEP 517) #1362
• Optional: Investigate / use Meson and meson-python instead of setuptools? See for example https://github.com/GNOME/meld (or any other Gnome Python app)
• Optional: Store the version (and gi_versions?) in setup.toml (see wip: build system refactoring draft #1312 or use importlib.metadata (py3.8+) to import the version (pkg_resources.get_distribution('ulauncher').version) only (better way imo)
• Optional: Try not to hard code the app id: https://github.com/GNOME/gnome-font-viewer/blob/518e33674b6f4fabf2558dd917ac1771bd088365/data/org.gnome.font-viewer.service.in#L2

Poetry has a PEP 517 build back-end (poetry-core), but it is not PEP 621 compliant (not pyproject.toml), and Poetry also comes with a lot of dependencies, and they did do that weird breaking change in 1.2, where it threw an error 5% of the time intentionally in CI, so I am very reluctant to become dependent on poetry.