add Python 3.12 trove classifier

Author: timgraham

setup.cfg

@@ -20,6 +20,7 @@ classifiers =
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
 project_urls =
     Source = https://github.com/cockroachdb/django-cockroachdb
     Tracker = https://github.com/cockroachdb/django-cockroachdb/issues