[Fix] Fix docs build error (#2739)

.readthedocs.yml

 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.8"
+
 formats:
   - epub
 
 python:
-  version: 3.7
   install:
     - requirements: requirements/docs.txt
     - requirements: requirements/readthedocs.txt