throw away all node attributes, except for 'label

def _preprocess_nodes_for_pydot(nodes_with_data):
    """throw away all node attributes, except for 'label'"""
    for (node_id, attrs) in nodes_with_data:
        if 'label' in attrs:
            yield (quote_for_pydot(node_id),
                   {'label': quote_for_pydot(attrs['label'])})
        else:
            yield (quote_for_pydot(node_id), {})