It is not only py2 as build on py3 is also affected, e.g.

https://travis-ci.com/github/zopefoundation/ZODB/jobs/499826281

And even though Sphinx is going to be ready for newer docutils,
sphinx_rtd_theme requires docutils<0.17 for now:

https://github.com/readthedocs/sphinx_rtd_theme/blob/2026bb61ddf85a672141127b162d6f9181603725/setup.py#L119-L122