title: Pytube only works periodically (KeyError: 'assets')
body: <p>Five out of ten times Pytube will send me this error when attempting to run my small testing script.</p>
  <p>Here's the script:</p>
  <pre><code>import pytube
  import urllib.request
  
  
  from pytube import YouTube
  yt = YouTube('https://www.youtube.com/watch?v=3NCyD3XoJgM')
  
  print('Youtube video title is: ' + yt.title + '! Downloading now!')
  </code></pre>
  <p>Here's what I get:</p>
  <pre><code>Traceback (most recent call last):
    File &quot;youtube.py&quot;, line 6, in &lt;module&gt;
      yt = YouTube('https://www.youtube.com/watch?v=3NCyD3XoJgM')
    File &quot;C:\Users\test\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-packages\Python38\site-packages\pytube\__main__.py&quot;, line 91, in __init__
      self.prefetch()
    File &quot;C:\Users\test\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-packages\Python38\site-packages\pytube\__main__.py&quot;, line 183, in prefetch
      self.js_url = extract.js_url(self.watch_html)
    File &quot;C:\Users\test\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-packages\Python38\site-packages\pytube\extract.py&quot;, line 143, in js_url
      base_js = get_ytplayer_config(html)[&quot;assets&quot;][&quot;js&quot;]
  KeyError: 'assets'
  </code></pre>
  <p>I am very confused. I attempted to reinstall Python plus pytube but I can't seem to remedy this issue. It's increasingly perplexing that the script works half of the time, but not the other half.</p>