Resource paths are passed to ``pkg_resources.resource_string`` and similar no longer accept paths that traverse parents, that begin with a leading ``/``. Violations of this expectation raise DeprecationWarnings and will become errors. Additionally, any paths that are absolute on Windows are strictly disallowed and will raise ValueErrors.
Added the ``build_meta:__legacy__`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.