Add build os to docs yml

Signed-off-by: Beat Buesser <[email protected]>

Author: beat-buesser

readthedocs.yml

@@ -1,11 +1,16 @@
 version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: 3.9
+
+python:
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs
+
 sphinx:
   configuration: docs/conf.py
-os: ubuntu-20.04
-python:
-    version: 3.6
-    install:
-      - method: pip
-        path: .
-        extra_requirements:
-          - docs