input: Write a function to create a web API using Flask in Python that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS". The endpoint should be accessible at `/datetime`.
  assert get_datetime()=='2023-10-05 14:30:45'
output: 
  from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      now = datetime.now()
      return now.strftime("%Y-%m-%d %H:%M:%S")
  