[backport][CI] Use Python 3.10 to build docs (dmlc#10383) (dmlc#10469)

Co-authored-by: Philip Hyunsu Cho <[email protected]>

Author: trivialfis

.readthedocs.yaml

@@ -12,7 +12,7 @@ submodules:
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.8"
+    python: "3.10"
   apt_packages:
     - graphviz
     - cmake