Update readthedocs config

RTD deprecated `build.image` in favor of `build.os`, which we must
update. See https://blog.readthedocs.com/use-build-os-config/ for
details.

.readthedocs.yml

@@ -1,15 +1,11 @@
 version: 2
 
 build:
-  image: latest
+  os: "ubuntu-22.04"
   apt_packages:
     - libnuma1
-
-python:
-  version: 3.8
-  install:
-    - method: pip
-      path: .
+  tools:
+    python: "3.9"
 
 conda:
   environment: conda/environments/builddocs.yml