Update sphinx version and config

intersphinx-mapping needed adjustment, plus there is some sort
of incompatibility between sphinx 7.0.0 and sphinx-rtd-theme,
so pin sphinx below that point.  Allegedly it is fixed in
more recent sphinx-rtd-theme installations, but we do not pin
that package and it still breaks, so punting that to be
solved some other time.

docs/source/conf.py

@@ -166,5 +166,5 @@
 ]
 
 
-# Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {"https://docs.python.org/": None}
+# Configuration for intersphinx: refer to the Python standard library.
+intersphinx_mapping = {"python": ("https://docs.python.org/3", None)}

docs/source/requirements.txt

@@ -1,3 +1,3 @@
-Sphinx>=1.5.3
+Sphinx<7.0.0
 sphinx-prompt>=1.0.0
 sphinx_rtd_theme