gh-125895: Fix static asset location for sphinx-notfound-page (GH-147984)

(cherry picked from commit 80ab6d9)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: StanFromIreland

Doc/conf.py

@@ -570,6 +570,17 @@
 stable_abi_file = 'data/stable_abi.dat'
 threadsafety_file = 'data/threadsafety.dat'
 
+# Options for notfound.extension
+# -------------------------------
+
+if not os.getenv("READTHEDOCS"):
+    if language_code:
+        notfound_urls_prefix = (
+            f'/{language_code.replace("_", "-").lower()}/{version}/'
+        )
+    else:
+        notfound_urls_prefix = f'/{version}/'
+
 # Options for sphinxext-opengraph
 # -------------------------------