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

AlexanderVNikitin · AlexanderVNikitin · commit 05a3ecea8ecd · 2023-12-16T12:21:05.000+02:00
diff --git a/readthedocs.yml b/readthedocs.yml
@@ -1,6 +1,7 @@
 version: 2
 
 build:
+   os: ubuntu-20.04
    image: latest
 
 python:
