input: Write a function to create a Flask web API that returns the current date and time when accessed at the '/datetime' endpoint.
  
  assert get_datetime()=='2023-10-10 12:34:56'
  assert get_datetime()=='2023-11-11 11:11:11'
  
  Note: The exact timestamp will vary depending on when the function is called.
output: 
  from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      now = datetime.now()
      return jsonify({'datetime': now.strftime('%Y-%m-%d %H:%M:%S')})
  