Fix intersphinx mapping

The map is supposed to be a dict of tuples, see
https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html



Signed-off-by: Stephen Kitt <steve@sk2.org>
Part-of: <!31>

doc/source/conf.py

@@ -264,4 +264,4 @@ texinfo_documents = [
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+intersphinx_mapping = {'python': ('http://docs.python.org/3', None)}