input: Write a python function to create a simple HTTP server using `http.server` and `socketserver` that serves on port 80.
  assert start_server() == "serving at port 80"
output: def start_server():
      import http.server
      import socketserver
  
      PORT = 80
  
      class Handler(http.server.SimpleHTTPRequestHandler):
          pass
  
      with socketserver.TCPServer(("", PORT), Handler) as httpd:
          print(f"serving at port {PORT}")
          httpd.serve_forever()
  
      return f"serving at port {PORT}"