Here's the failure, for context:

$ luci-py/appengine/swarming/server/task_request_test.py
Traceback (most recent call last):
  File "server/task_request_test.py", line 20, in <module>
    from test_support import test_case
  File ".../luci-py/appengine/swarming/test_support/test_case.py", line 15, in <module>
    import webtest
  File ".../luci-py/appengine/components/../third_party_local/webtest/__init__.py", line 9, in <module>
    from webtest.app import TestApp
  File ".../luci-py/appengine/components/../third_party_local/webtest/app.py", line 30, in <module>
    from webtest.response import TestResponse
  File ".../luci-py/appengine/components/../third_party_local/webtest/response.py", line 5, in <module>
    from webtest import forms
  File ".../luci-py/appengine/components/../third_party_local/webtest/forms.py", line 6, in <module>
    from bs4 import BeautifulSoup
  File ".../luci-py/appengine/components/../third_party_local/bs4/__init__.py", line 30, in <module>
    from .builder import builder_registry, ParserRejectedMarkup
  File ".../luci-py/appengine/components/../third_party_local/bs4/builder/__init__.py", line 311, in <module>
    from . import _html5lib
  File ".../luci-py/appengine/components/../third_party_local/bs4/builder/_html5lib.py", line 57, in <module>
    class TreeBuilderForHtml5lib(html5lib.treebuilders._base.TreeBuilder):
AttributeError: 'module' object has no attribute '_base'

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/bd5deb1d9cde6e64051495efe2398973565372ab

commit bd5deb1d9cde6e64051495efe2398973565372ab
Author: Peter Ward <flowblok@chromium.org>
Date: Tue Feb 27 00:52:36 2018

Update beautifulsoup from 4.3.1 to 4.6.0.

Bug:  816358 
Change-Id: I474c8087dab1401b35e0c9fd0183e7bd78920e60
Reviewed-on: https://chromium-review.googlesource.com/936884
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: flowblok <flowblok@chromium.org>

[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/COPYING.txt
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/README.swarming
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/__init__.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/builder/__init__.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/builder/_html5lib.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/builder/_htmlparser.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/builder/_lxml.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/dammit.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/diagnose.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/element.py
[modify] https://crrev.com/bd5deb1d9cde6e64051495efe2398973565372ab/appengine/third_party_local/bs4/testing.py