readthedocs: update config with OS definition

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

readthedocs.yml

diff --git a/readthedocs.yml b/readthedocs.yml
index 519abe1..c591305 100644
--- a/readthedocs.yml
+++ b/readthedocs.yml
@@ -19,14 +19,16 @@
 
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "mambaforge-22.9"
+
 sphinx:
   configuration: docs/conf.py
 
 formats: []
 
-python:
-  version: "3.7"
-
 conda:
   environment: docs/environment.yml