Revert "Stop specifying OS version in our RTD config"

Had it backwards, the problem is with Drutopia under-specifying so the
simple way that used to work no longer does.  Ridiculous that
ReadTheDocs makes us specify the build operating system as if we
flipping care 99% of the time.

This reverts commit 6611ee36a1.

.readthedocs.yml

@@ -1,7 +1,11 @@
 version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
 sphinx:
   configuration: conf.py
 python:
-  version: 3
   install:
     - requirements: requirements.txt