Update GHA config for Python 3.12 final

I think we can just do this now Python 3.12 is stable.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

.github/workflows/tox.yml

@@ -17,7 +17,7 @@ jobs:
           - "3.9"
           - "3.10"
           - "3.11"
-          - "3.12.0-alpha - 3.12.0"
+          - "3.12"
         include:
           - os: "ubuntu-latest"
           - os: "ubuntu-20.04"