Thank you, that worked -- and should have been documented in the help output. All the sphinx dependencies were inequalities, and rebuilding worked.
The devguide only says "You are expected to have installed a recent version of Sphinx on your system or in a virtualenv,". Could we add
"We currently recommend 'python -m pip install sphinx==1.3.6' as the easiest way to install and the best version for most people to install. See https://bugs.python.org/issue26638 for why."
(I am assuming that pip works on all systems, not just Windows.) The sentence finishes with "so that the Makefile can find the sphinx-build command." On Windows. this is most easily done by installing Sphinx with a Python version that was added to PATH when installed.