migrate from setup.py to pyproject.toml

[deltragon]

Description

This PR switches configuration from setup.py to pyproject.toml.

This is somewhat of a prerequisite in the plan discussed in https://github.com/slgobinath/SafeEyes/discussions/589. Most of the tools there (type checkers, nox/tox, ruff) expect to be configured with a pyproject.toml file. This documentation also "strongly recommends" using a pyproject.toml file in some form.

In theory, it would be enough to just use the [build-backend] table, and keep the setup.py mostly as-is. However, when a pyproject.toml file is detected, it also enables build isolation automatically.

Build isolation means that the way .mo files were generated needs to be changed (it is now a custom build step), and the data_files key is deprecated, so I decided to properly migrate everything to pyproject.toml.

The only thing this should have an impact on is building the release. I have tested locally, and using python -m build properly builds the .whl and .tar.gz as before, with identical contents apart from some metadata differences.