The source project of this merge request has been removed.
Set the default build-backend to setuptools.build_meta:__legacy__
This is the defined fallback behaviour when build-backend is not defined.
This fixes a crash when converting packages where build-backend is not defined such as the forum-dl PyPI package.
See-also: https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#fallback-behaviour