Add openstack-tox-py312 job

Use pyenv flag for the ensure-python role since python3.12 isn't
available yet as distro pkg on bookworm.

Change-Id: If42cb167c3395e530e3dcd0090bbca86023ac738

zuul.d/jobs.yaml

@@ -348,6 +348,24 @@
       bindep_profile: test py311
       python_version: '3.11'
 
+- job:
+    name: openstack-tox-py312
+    parent: openstack-tox
+    nodeset: debian-bookworm
+    timeout: 2400
+    description: |
+
+      Run unit tests for an OpenStack Python project under cPython
+      version 3.12.
+
+      Uses tox with the ``py312`` environment.
+    irrelevant-files: *common-irrelevant-files
+    vars:
+      tox_envlist: py312
+      bindep_profile: test py312
+      python_use_pyenv: True
+      python_version: '3.12'
+
 - job:
     name: openstack-tox-py311-arm64
     parent: openstack-tox-py311