这个问题是由PyTube自身库中的一个小问题导致的（当然，你得确保已经开了*），目前已经修复了。

用 'pip install pytube --upgrade'更新一下你的库，再试试看。

This issue was induced by a problem with the PyTube library. There is an issue about it here: https://github.com/pytube/pytube/issues/495

It is already fixed and a new release was published. 

Using  'pip install pytube --upgrade' and run your code again.