use build.os due to readthedocs/readthedocs.org#8861 (comment)

AlexanderVNikitin · AlexanderVNikitin · commit bfc93d93c612 · 2023-12-16T19:49:17.000+02:00
diff --git a/readthedocs.yml b/readthedocs.yml
@@ -1,6 +1,9 @@
 version: 2
 
 build:
+   os: ubuntu-20.04
+   tools:
+      python: "3.8"
    image: latest
 
 python:
