input: Write a function to create a simple HTTP server that responds with "Hello, world!".
  assert create_server().server_address == ('localhost', 8002)
  assert create_server().RequestHandlerClass.__name__ == 'MyHandler'
output: def create_server():
      import http.server
      import socketserver
  
      class MyHandler(http.server.SimpleHTTPRequestHandler):
          def do_GET(self):
              self.send_response(200)
              self.send_header("Content-type", "text/plain")
              self.end_headers()
              self.wfile.write(b"Hello, world!")
  
      PORT = 8002
      Handler = MyHandler
  
      with socketserver.TCPServer(("localhost", PORT), Handler) as httpd:
          return httpd