input: def get_current_datetime_via_web_api() -> str:
      """ Create a web API using Python and Flask to return the current date and time.
      Example:
      The Flask application should have an endpoint '/datetime' that returns the current time in the format 'YYYY-MM-DD HH:MM:SS'.
      When accessed via a browser or an HTTP client, it would display the current date and time.
      """
      from flask import Flask
      import datetime
  
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
      if __name__ == '__main__':
          app.run()
output:     return "This function is not intended to be called directly. Please run the Flask application and access the '/datetime' endpoint."