Set Ubuntu and python in .readthedocs.yaml

Issue-ID: DOC-811
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I0c850b2a55ac9d302b28701409e8e1361afea2c7
(cherry picked from commit 1bf96f951e892e11e597c180a099871e32ddba7d)
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 9c22c0b..dfd1cd9 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -21,18 +21,16 @@
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 # Required
+
 version: 2
-
-formats:
-  - htmlzip
-
 build:
-  image: latest
+  os: ubuntu-20.04
+  tools:
+    python: "3.8"
 
 python:
-  version: 3.7
   install:
-    - requirements: docs/requirements-docs.txt
+  - requirements: docs/requirements-docs.txt
 
 sphinx:
   configuration: docs/conf.py