use pyproject.toml instead of setup.py

[rffontenelle]

pip is now outputting deprecation warning when installing projects setup.py-based in favor of pyproject.toml :

DEPRECATION: sphinx-intl is being installed using the legacy 'setup.py install' method, because it does not have a 'pyproject.toml' and the 'wheel' package is not installed. pip 23.1 will enforce this behaviour change. A possible replacement is to enable the '--use-pep517' option. Discussion can be found at pypa/pip#8559

• Update docs where it says pip>=8 (e.g. installation) is supported because pip supports pyproject.toml since version 10.0
• flake8 does not support pyproject.toml