input: Write a python function to retrieve all rows from the 'book' table using pandas and a given database connection.
  assert isinstance(retrieve_books(conn), pd.DataFrame)
  assert set(retrieve_books(conn).columns) == {'id', 'title', 'author', 'year', 'genre'}
output: def retrieve_books(conn):
      import pandas as pd
      query = "SELECT * FROM book"
      df = pd.read_sql_query(query, conn)
      return df