Start testing Python 3.12

It is out and stable, so let's make sure we are compatible.

Author: CendioOssman

.github/workflows/test.yml

@@ -14,6 +14,7 @@ jobs:
           - 3.9
           - "3.10"
           - 3.11
+          - 3.12
       fail-fast: false
     steps:
       - uses: actions/checkout@v2