Replace intersphinx_mapping format removed in Sphinx 8.0.0

Fixed #1356

doc/source/conf.py

@@ -309,4 +309,4 @@ def write_section(specsection, secname, key, comment, output):
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}