Fix doc builds for intersphinx mapping. Supersedes #2659.

Author: jaraco

docs/conf.py

@@ -139,7 +139,7 @@
 ]
 
 # Allow linking objects on other Sphinx sites seamlessly:
-intersphinx_mapping = {
-    'python': ('https://docs.python.org/3', None),
-    'python2': ('https://docs.python.org/2', None),
-}
+intersphinx_mapping.update(
+    python=('https://docs.python.org/3', None),
+    python2=('https://docs.python.org/2', None),
+)