input: Write a python function to create a Flask app with routes for creating, reading, updating, and deleting a user.
  assert app.test_client().post('/users').data.decode() == 'Create a new user.'
  assert app.test_client().get('/users/1').data.decode() == 'Read user 1'
  assert app.test_client().put('/users/1').data.decode() == 'Update user 1'
  assert app.test_client().delete('/users/1').data.decode() == 'Delete user 1'
output: from flask import Flask, request
  
  def create_app():
      app = Flask(__name__)
  
      @app.route('/users', methods=['POST'])
      def create_user():
          return 'Create a new user.'
  
      @app.route('/users/<int:user_id>', methods=['GET'])
      def read_user(user_id):
          return f'Read user {user_id}'
  
      @app.route('/users/<int:user_id>', methods=['PUT'])
      def update_user(user_id):
          return f'Update user {user_id}'
  
      @app.route('/users/<int:user_id>', methods=['DELETE'])
      def delete_user(user_id):
          return f'Delete user {user_id}'
  
      return app
  
  app = create_app()