input: Write a function to convert a given SQL statement into a Python Pandas DataFrame. The function should execute the SQL query and return the resulting DataFrame.
  assert convert_sql_to_df("SELECT * FROM book;")!=None
  assert isinstance(convert_sql_to_df("SELECT * FROM book;"), pd.DataFrame)
output: def convert_sql_to_df(sql_query):
      import pandas as pd
      import sqlite3
      conn = sqlite3.connect('example.db')
      df = pd.read_sql_query(sql_query, conn)
      conn.close()
      return df