bump intersphinx python ref to py3

docs/source/conf.py

@@ -201,7 +201,7 b' html_additional_pages = {'
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'ipythondoc'
 
-intersphinx_mapping = {'python': ('http://docs.python.org/2/', None),
+intersphinx_mapping = {'python': ('http://docs.python.org/3/', None),
                        'rpy2': ('http://rpy.sourceforge.net/rpy2/doc-2.4/html/', None),
                        'traitlets': ('http://traitlets.readthedocs.org/en/latest/', None),
                        'jupyterclient': ('http://jupyter-client.readthedocs.org/en/latest/', None),