input: Write a python function to start a simple HTTP server on a given port.
  assert start_simple_http_server(8000) == "serving at port 8000"
  assert start_simple_http_server(8080) == "serving at port 8080"
output: def start_simple_http_server(port):
      import http.server
      import socketserver
  
      Handler = http.server.SimpleHTTPRequestHandler
  
      with socketserver.TCPServer(("", port), Handler) as httpd:
          return f"serving at port {port}"