diff --git a/.readthedocs.yml b/.readthedocs.yml
index 58afb426..d335c40d 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -5,6 +5,7 @@ formats: all
 build:
   os: ubuntu-20.04
   tools:
+    # Keep version in sync with tox.ini (docs and gh-actions).
     python: "3.10"
 
 python: