input: Write a function to start a simple HTTP server that serves a given HTML file.
  assert start_http_server('index.html') is None
  assert start_http_server('another_page.html') is None
output: import http.server
  import socketserver
  
  def start_http_server(html_file):
      class CustomHandler(http.server.SimpleHTTPRequestHandler):
          def do_GET(self):
              self.send_response(200)
              self.send_header("Content-type", "text/html")
              self.end_headers()
              with open(html_file, 'rb') as file:
                  self.wfile.write(file.read())
      
      PORT = 8000
      with socketserver.TCPServer(("", PORT), CustomHandler) as httpd:
          print("Serving at port", PORT)
          httpd.serve_forever()