doc: fix build error due to outdated intersphinx mapping

Newest versions of Sphinx changed the format for the
`intersphinx_mapping` option: https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping
Now requiring a tuple. Fix this so docs can be built again.

Signed-off-by: Antonio Torres <[email protected]>

docs/source/conf.py

@@ -155,4 +155,4 @@
 ]
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'https://docs.python.org/': None}
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}