Merge pull request #1024 from VWS-Python/new-style-rtd

Try new RTD format

Author: adamtheturtle

readthedocs.yaml

@@ -1,17 +1,16 @@
 version: 2
 
-# We do this because at the time of writing we need "image: testing" for Python
-# 3.9.
 build:
-  image: testing
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
 
 python:
   install:
      - method: pip
        path: .
        extra_requirements:
            - dev
-  version: 3.9
 
 sphinx:
   builder: html