My attempt at #3816 :)

This PR:

• Switches the project to use a pyproject.toml to centralize package setup, depedency grouping and tool settings
• Keeps using setuptools to build Cython-ized wheels, but simplifies the code greatly (see build_backend.py and setup.py and how it's driven by tox.ini)
• Removes the unused fonttools entrypoint script
• Set up setuptools_scm to read the package version from the Git tag, like many other projects, obviating bumpversion and similar tools
• Sets up tox to use the tox-uv package, to make full use of uv

You can build pure wheels with uv build --wheel and Cythonized packages with uv build -C FONTTOOLS_WITH_CYTHON=true.

TODO:

• Would need to switch tox over to tox-uv maybe.
• Find a replacement for the code that updates the Changelog on release.
• How to install manpages?