Find and load the build backend

def _build_backend():
    """Find and load the build backend"""
    ep = os.environ['PEP517_BUILD_BACKEND']
    mod_path, _, obj_path = ep.partition(':')
    try:
        obj = import_module(mod_path)
    except ImportError:
        raise BackendUnavailable
    if obj_path:
        for path_part in obj_path.split('.'):
            obj = getattr(obj, path_part)
    return obj