old_contents: #!/usr/bin/env python3
  from flask import Flask
  from flask import request
  
  
  app = Flask(__name__)
  
  
  @app.route('/', methods=['GET'])
  def hello_world():
      return 'Hi there'
  
  
  if __name__ == '__main__':
      app.run()
  
new_contents: #!python3
  from flask import Flask
  from flask import request
  
  
  app = Flask(__name__)
  
  
  @app.route('/', methods=['GET'])
  def hello_world():
      return 'Hi there'
  
  
  if __name__ == '__main__':
      app.run()
  
subject: use python3 from virtualenv