Update intersphinx_mapping to non-deprecated format (#1140)

Co-authored-by: Ludovico Bianchi <me@ludob.com>

docs/source/conf.py

@@ -379,7 +379,7 @@
 
 
 # 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)}
 
 rst_epilog = """
 .. |examples_reminder_text| replace:: The **examples/** directory refers to the location where the FOQUS examples were installed, as described in :ref:`install_examples`.