How can I put an intersphinx link to an arbitrary method in the standard library documentation?

I am trying to use Sphinx to document a project, but I cannot figure out how to use intersphinx. I use this line:

:py:meth:`math.sin`

to add the link, but in the output, it shows up bold, not as a link. Although that line does not work, both the following do:

:py:meth:`dict.items`
:py:class:`zipfile.ZipFile`

My intersphinx_mapping value in the conf.py file is:

intersphinx_mapping = {'python':('http://docs.python.org/2.7', None)}