[4.0.x] Refs #33247 -- Corrected configuration for Read The Docs.

This pins Sphinx version, because the default Sphinx version used by
RTD is not compatible with Python 3.8+.

This also, sets Python 3.8 for RTD builds which is compatible with all
current versions of Django.

Thanks to Mariusz Felisiak for the suggestion.
Backport of 447b6c866f from main

.readthedocs.yml

@@ -1,17 +1,20 @@
 # Configuration for the Read The Docs (RTD) builds of the documentation.
 # Ref: https://docs.readthedocs.io/en/stable/config-file/v2.html
-# Note python.install.requirements is not currently required, as Sphinx is
-# preinstalled and spelling checks not performed by RTD.
+# The python.install.requirements pins the version of Sphinx used.
 version: 2
 
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.10"
+    python: "3.8"
 
 sphinx:
   configuration: docs/conf.py
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 formats:
   - epub
   - pdf

docs/requirements.txt

@@ -1,3 +1,3 @@
 pyenchant
-sphinx
+Sphinx>=3.1.0
 sphinxcontrib-spelling