input: Write a function to provide a command-line interface for downloading subtitles for movies and TV shows. The function should take one or more release paths as arguments and allow specifying the language and subtitle site. The available commands are "download", "exist", "test", "list", "help", and "version". The function should return the appropriate output based on the command.
  assert download_subtitles(["path/to/movie1", "path/to/movie2"], engines=["engine1"], lang="en")==None
  assert is_subtitles_exist(["path/to/movie1", "path/to/movie2"], engines=["engine1"], lang="en")==None
  assert test_engines(["engine1", "engine2"])==None
  assert SUBTITLE_SITE_LIST.keys()==["engine1", "engine2", "engine3"]
output: def download_subtitles(release_paths, engines=None, lang=None):
      pass
  
  def is_subtitles_exist(release_paths, engines=None, lang=None):
      pass
  
  def test_engines(engines):
      pass
  
  def list_subtitle_sites():
      return {"engine1", "engine2", "engine3"}
  
  def help_command():
      pass
  
  def version_command():
      pass