diff --git a/.readthedocs.yml b/.readthedocs.yml
index 44949ea239..7c6c25541a 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -5,6 +5,13 @@
 # Required
 version: 2
 
+build:
+  os: "ubuntu-20.04"
+  apt_packages:
+    - python3-six
+  tools:
+    python: "3.9"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: doc/conf.py
@@ -12,8 +19,6 @@ sphinx:
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: []
 
-# Optionally set the version of Python and requirements required to build your docs
-# python:
-#   version: 3.7
-#   install:
-#     - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: doc/sphinx/requirements.txt
diff --git a/doc/sphinx/requirements.txt b/doc/sphinx/requirements.txt
index 4555a94d30..44c187880d 100644
--- a/doc/sphinx/requirements.txt
+++ b/doc/sphinx/requirements.txt
@@ -1,4 +1,4 @@
 docutils==0.16
-Sphinx==3.4.3
-sphinx_rtd_theme
-six
+sphinx==3.4.3
+sphinx_rtd_theme==1.0.0
+six==1.16.0