Update readthedocs config (#999)

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

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)

Approvers:
  - https://github.com/jakirkham

URL: #999

.readthedocs.yml

@@ -1,12 +1,11 @@
 version: 2
 
 build:
-  image: latest
-  apt_packages:
-    - libnuma1
+  os: "ubuntu-22.04"
+  tools:
+    python: "mambaforge-22.9"
 
 python:
-  version: 3.8
   install:
     - method: pip
       path: .