input: Write a function to create a web API using Python and Flask that returns the current date and time when accessed. The function should define a route '/datetime' that, when accessed, returns the current date and time in the format "YYYY-MM-DD HH:MM:SS".
  assert get_datetime().split()[0]==datetime.datetime.now().strftime("%Y-%m-%d")
  assert get_datetime()==datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
output: 
  @app.route('/datetime')
  def get_datetime():
      return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  