docs: Update intersphinx_mapping location, add missing downstreams

Signed-off-by: Brian Harring <ferringb@gmail.com>

doc/conf.py

@@ -51,6 +51,7 @@
     "sphinx.ext.ifconfig",
     "sphinx.ext.graphviz",
     "sphinx.ext.viewcode",
+    "seed_intersphinx_mapping",
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -241,7 +242,14 @@
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {"http://docs.python.org/": None}
+intersphinx_mapping = {
+    "lazy-object-proxy": ("https://python-lazy-object-proxy.readthedocs.io/en/latest/", None),
+    "python": ("https://docs.python.org/3", None),
+}
+
+# directive to seed_intersphinx_mapping; we use pyproject, thus read that.
+pkg_requirements_source = "pyproject"
+
 autodoc_default_flags = [
     "members",
     "show-inheritance",