input: from typing import List
  
  def current_datetime_api(app: app) -> str:
      """Create a web API with Flask to give the current date and time.
      The API should be accessible at the route '/datetime' and should return 
      the current date and time in the format 'YYYY-MM-DD HH:MM:SS'.
  
      Example:
      For the given Flask app, the route '/datetime' should return the current date and time.
      >>> current_datetime_api(app)
      '2023-10-05 12:34:56'
      """
output:     from datetime import datetime
  
      @app.route('/datetime')
      def get_current_datetime():
          now = datetime.now()
          return now.strftime('%Y-%m-%d %H:%M:%S')