.readthedocs.yml: update the requirements

Fix an error:
  This project needs at least Sphinx v2.4.4.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

.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

doc/sphinx/requirements.txt

 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