input: Write a python function to start a simple HTTP server on a specified port using the http.server and socketserver modules.
  assert start_server(8000) == "serving at port 8000"
  assert start_server(8080) == "serving at port 8080"
  assert start_server(3000) == "serving at port 3000"
output: def start_server(port):
      import http.server
      import socketserver
  
      class Handler(http.server.SimpleHTTPRequestHandler):
          pass
  
      with socketserver.TCPServer(("", port), Handler) as httpd:
          return f"serving at port {port}"