input: Write a function to create a web API using Python and Flask that returns the current date and time. If the client makes a request to the '/datetime' endpoint, the API should respond with the current date and time in the format "YYYY-MM-DD HH:MM:SS".
  assert get_datetime()=="2023-10-05 14:30:45"
  assert get_datetime()=="2023-10-05 14:31:00"
  assert get_datetime()=="2023-10-05 14:31:15"
output: 
  from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime', methods=['GET'])
  def get_datetime():
      now = datetime.now()
      return now.strftime("%Y-%m-%d %H:%M:%S")
  