First of all, the latest version pytube (which is installed with pip install pytube, today is version 6.1.5 , so you should probably look at this PyPI version page. The brief documentation on the PyPI page says nothing about the class Client, instead the main API Class YouTube, as shown in this excerpt:
from pytube import YouTube
yt = YouTube("http://www.youtube.com/watch?v=Ik-RsDGPI5Y")
pprint(yt.get_videos())
"PyTube" , , . , PyPI.