docs: Fix RTD configuration for v2 (#4032)

RTD recently cut off old configuration file style and these additions
were needed.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

.readthedocs.yml

@@ -2,9 +2,20 @@
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 
+# Copyright Contributors to the OpenImageIO project.
+# SPDX-License-Identifier: Apache-2.0
+# https://github.com/AcademySoftwareFoundation/OpenImageIO
+
+
 # Required
 version: 2
 
+# Set the OS, Python version and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: src/doc/conf.py
@@ -16,6 +27,5 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
     - requirements: src/doc/requirements.txt