bump readthedocs conf

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

.readthedocs.yml

diff --git a/.readthedocs.yml b/.readthedocs.yml
index 0b154b6..bdc3ce5 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -5,6 +5,11 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.7"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
@@ -14,7 +19,6 @@
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: "3.7"
   install:
     - requirements: docs/requirements.txt