Move to Python version 3.8 for readthedocs build.

PiperOrigin-RevId: 469168906
Change-Id: Ie54f6f0c9d4fc34cfad317ac671b22a6ba777c44

readthedocs.yml

@@ -18,15 +18,10 @@ python:
     - requirements: docs/requirements.readthedocs.txt
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-20.04
   tools:
-    python: "3.10"
+    python: "3.8"
   jobs:
-    pre_build:
-      # Fix for: https://stackoverflow.com/questions/65640476/pip3-on-python3-9-fails-on-htmlparser-object-has-no-attribute-unescape
-      - python3 -m pip install --upgrade setuptools
-      - python3 -m pip install --upgrade pip
-      - python3 -m pip install --upgrade distlib
     post_build:
       - echo "Running post-build commands."
       - echo `date`