input: Write a Python function to start a Flask web server that serves the current datetime at the '/datetime' endpoint.
  assert get_datetime() != ""
  assert isinstance(get_datetime(), str)
  assert len(get_datetime()) == 19
output: from flask import Flask
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      return datetime.now().strftime('%Y-%m-%d %H:%M:%S')
  
  if __name__ == '__main__':
      app.run(debug=True)