In the context of #4456, I was trying to move the documentation build on travis to python 3.5, but found that sphinx raised a couple of warnings that do not appear on python 2 (see https://travis-ci.org/astropy/astropy/builds/101435099). I confirmed locally that one gets

io/votable/references.txt:4: WARNING: py:class reference target not found: SyntaxWarning
io/votable/references.txt:4: WARNING: py:class reference target not found: SyntaxWarning
io/votable/references.txt:4: WARNING: py:class reference target not found: SyntaxWarning
io/votable/references.txt:4: WARNING: py:class reference target not found: RuntimeWarning
io/votable/references.txt:4: WARNING: py:class reference target not found: ValueError

This seems to be in the context of creating the description for the classes [1] where the links to those warnings cannot be found and are thus missing. Could there be something wrong in the astropy helper setup? (e.g., astropy_helpers/sphinx/conf.py -- definitely not obvious to me...).

[1] http://docs.astropy.org/en/latest/io/votable/api_exceptions.html#exception-utilities