title: Glitch in html5lib?
body: <p>I'm getting this error. Is it a bug or is it a code error? What does it mean?</p>
  <pre><code>Traceback (most recent call last):
    File &quot;isc.py&quot;, line 8, in &lt;module&gt;
      import requests, os, sys, bs4
    File &quot;/opt/virtualenvs/python3/lib/python3.8/site-packages/bs4/__init__.py&quot;, line 30, in &lt;module&gt;
      from .builder import builder_registry, ParserRejectedMarkup
    File &quot;/opt/virtualenvs/python3/lib/python3.8/site-packages/bs4/builder/__init__.py&quot;, line 314, in &lt;module&gt;
      from . import _html5lib
    File &quot;/opt/virtualenvs/python3/lib/python3.8/site-packages/bs4/builder/_html5lib.py&quot;, line 70, in &lt;module&gt;
      class TreeBuilderForHtml5lib(html5lib.treebuilders._base.TreeBuilder):
  AttributeError: module 'html5lib.treebuilders' has no attribute '_base'
  </code></pre>
  <p>(i upgraded html5lib bs4 and requests)</p>