Declare / update RTD build environment

This change adds the "build" configuration option for ReadTheDocs, and
requires Ubuntu 22.04 LTS.

Python verison is also included in the "build" section, as old way of
declaration is deprecated. We use this opportunity to bump Python to
v3.9 (matching current debian stable) as 3.7 nears its EOL.

The change is prompted by build failure, after requirements pulled in
urllib3 v2.0.x, which removed support for OpenSSL versions earlier
than 1.1.1. Build error indicated openssl 1.0.2n which would imply we
might even have been on 18.04 (?). Fixes #954

.readthedocs.yml

@@ -5,6 +5,11 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.9"
+
 submodules:
   include: all
   recursive: true
@@ -22,6 +27,5 @@ formats: all
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
-    - requirements: requirements.txt
+    - requirements: requirements.txt