Docs: fix 404 page prefix configuration

The default is /en/latest/, but current docs hosting doesn't use this pattern. There's one 404 page for whole docs subdomain, so there's no need to provide a prefix per-version and/or per-language.

Author: m-aciek

Doc/conf.py

@@ -548,6 +548,11 @@
 refcount_file = 'data/refcounts.dat'
 stable_abi_file = 'data/stable_abi.dat'
 
+# Options for notfound.extension
+# ------------------------------
+if not os.getenv("READTHEDOCS"):  # RTD uses different URLs patterns
+    notfound_urls_prefix = '/3/'
+
 # Options for sphinxext-opengraph
 # -------------------------------