$ ./python Lib/test/test_pydoc.py
...
======================================================================
ERROR: test_mixed_case_module_names_are_lower_cased (__main__.PydocDocTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_pydoc.py", line 462, in test_mixed_case_module_names_are_lower_cased
    self.assertIn('xml.etree.elementtree', doc_link)
  File "/home/serhiy/py/cpython/Lib/unittest/case.py", line 1102, in assertIn
    if member not in container:
TypeError: argument of type 'NoneType' is not iterable

----------------------------------------------------------------------
Ran 49 tests in 5.716s

FAILED (errors=1, skipped=3)
Traceback (most recent call last):
  File "Lib/test/test_pydoc.py", line 1106, in <module>
    test_main()
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 2071, in decorator
    return func(*args)
  File "Lib/test/test_pydoc.py", line 1100, in test_main
    PydocWithMetaClasses,
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 1940, in run_unittest
    _run_suite(suite)
  File "/home/serhiy/py/cpython/Lib/test/support/__init__.py", line 1905, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_pydoc.py", line 462, in test_mixed_case_module_names_are_lower_cased
    self.assertIn('xml.etree.elementtree', doc_link)
  File "/home/serhiy/py/cpython/Lib/unittest/case.py", line 1102, in assertIn
    if member not in container:
TypeError: argument of type 'NoneType' is not iterable