readthedocs: Specify OS, tools

This is required. The error message is misleading [1].

[1] readthedocs/readthedocs.org#8912

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

.readthedocs.yaml

@@ -1,6 +1,9 @@
 ---
 version: 2
 build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.11"
   jobs:
     post_checkout:
       - git fetch --unshallow