input: Write a function to create a web API using Python and Flask that returns the current date and time in the format "YYYY-MM-DD HH:MM:SS". The function should define a route '/datetime' and use Flask's `@app.route` decorator to expose this endpoint. The main script should run the Flask application if executed directly.
  assert get_datetime()=="2023-01-01 12:34:56"
  assert get_datetime()=="2023-12-31 23:59:59"
  assert get_datetime()=="2024-02-29 00:00:00"
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")