input: Write a python function to create a simple Flask app that returns "Hello world!" on the root route.
  assert hello() == 'Hello world!'
  assert app.url_map.__str__().find("HEAD /") != -1
  assert app.url_map.__str__().find("HEAD /static/<path:filename>") != -1
output: from flask import Flask
  
  app = Flask(__name__)
  
  @app.route('/')
  def hello():
      return 'Hello world!'