input: Write a function to create a web API using Flask that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS".
  assert get_datetime()==<current_date_time>
output: 
  from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime', methods=['GET'])
  def get_datetime():
      current_datetime = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
      return jsonify({"datetime": current_datetime})
  