input: 
  from flask import Flask
  import datetime
  
  def start_web_api():
      """Create a web API with Python and Flask to give the current date and time.
      When accessing the endpoint /datetime, the API should return the current date and time in the format "YYYY-MM-DD HH:MM:SS".
      >>> app = start_web_api()
      >>> client = app.test_client()
      >>> response = client.get('/datetime')
      >>> assert response.status_code == 200
      >>> assert len(response.data) > 0
      """
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
      return app
  
  if __name__ == '__main__':
      app = start_web_api()
      app.run()
  
output:     app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
      return app