Include ``pyproject.toml`` in source distribution by default. Projects relying on the previous behavior where ``pyproject.toml`` was excluded by default should stop relying on that behavior or add ``exclude pyproject.toml`` to their MANIFEST.in file.
Setuptools once again declares 'setuptools' in the ``build-system.requires`` and adds PEP 517 build support by declaring itself as the ``build-backend``. It additionally specifies ``build-system.backend-path`` to rely on itself for those builders that support it.