Update pyproject.toml with Poetry v2.0 (#499)

Migrate from [tool.poetry] to [project]:

• https://python-poetry.org/blog/announcing-poetry-2.0.0
• https://python-poetry.org/docs/pyproject/
• https://python-poetry.org/docs/faq/#migrate-pep621-new-static

Update website, changelog urls (#433 (closed))

Use dynamic Poetry classifiers and dependencies, more convenient but less generic to pyproject.toml project

Change classifier to Operating System :: Unix, since Silkaj is not supported on Windows

Move pydiscourse to optional-dependencies, since Poetry extra is deprecated

Set main authors, with full names, sorted by importance of their contributions

Close #499 (closed).