Add intersphinx.

docs/conf.py

@@ -26,7 +26,13 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.doctest']
+extensions = [
+    'sphinx.ext.intersphinx',
+    ]
+
+intersphinx_mapping = {
+    'python': ('http://docs.python.org/dev', None),
+}
 
 doctest_global_setup = """
 import os