Running `make pydoc-topics` in Doc/ on master fails with

$ make pydoc-topics
sphinx-build -b pydoc-topics -d build/doctrees -D latex_elements.papersize=  . build/pydoc-topics 
Running Sphinx v1.6.1
making output directory...
loading pickled environment... not yet created
building [mo]: targets for 0 po files that are out of date
building [pydoc-topics]: all pydoc topics
updating environment: 466 added, 0 changed, 0 removed
reading sources... [100%] whatsnew/index                                                                                                                                                        
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
/Users/jzijlstra-mpbt/py/venvs/venv36/lib/python3.6/site-packages/sphinx/application.py:444: RemovedInSphinx17Warning: app.status_iterator() is now deprecated. Use sphinx.util.status_iterator() instead.
  RemovedInSphinx17Warning)
/Users/jzijlstra-mpbt/py/venvs/venv36/lib/python3.6/site-packages/sphinx/application.py:425: RemovedInSphinx17Warning: app._display_chunk() is now deprecated. Use sphinx.util.display_chunk() instead.
  RemovedInSphinx17Warning)
building topics... [  1%] assert                                                                                                                                                                
Exception occurred:
  File "/Users/jzijlstra-mpbt/py/venvs/venv36/lib/python3.6/site-packages/sphinx/builders/__init__.py", line 129, in create_translator
    assert translator_class, "translator not found for %s" % self.__class__.__name__
AssertionError: translator not found for PydocTopicsBuilder
The full traceback has been saved in /var/folders/nh/ctlmlrjs6sx561k27fgn4btm0000gp/T/sphinx-err-_9ox_985.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [build] Error 1


I ran into this because some code in Lib/pydoc.py told me to run make pydoc-topics as part of the patch for issue 30406.