input: Write a python function to retrieve a GraphQL query string for fetching books data.
  assert books_query() == '''
      query {
          books {
              id
              title
              author
              published_date
          }
      }
      '''
output: def books_query():
      return '''
      query {
          books {
              id
              title
              author
              published_date
          }
      }
      '''