Pinning doc version numbers.

.readthedocs.yaml

@@ -30,6 +30,6 @@ sphinx:
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#   install:
-#     - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -1,4 +1,3 @@
-# Defining the exact version will make sure things don't break
-sphinx
-sphinx_rtd_theme
-readthedocs-sphinx-search
+sphinx==5.3.0
+sphinx_rtd_theme==1.1.1
+readthedocs-sphinx-search==0.1.1