A four-part version fails with sphinx_rtd_theme:
https://github.com/readthedocs/sphinx_rtd_theme/issues/1343 While
this is a problem with sphinx_rtd_theme, the easiest solution is to
limit Sphinx version (sphinx_rtd_theme updates slowly)
This should be reverted after either sphinx releases again with a
more normal version number, or sphinx_rtd_theme updates.