add Python 3.13 trove classifier

Author: timgraham

setup.cfg

@@ -19,6 +19,7 @@ classifiers =
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
     Programming Language :: Python :: 3.12
+    Programming Language :: Python :: 3.13
 project_urls =
     Source = https://github.com/cockroachdb/django-cockroachdb
     Tracker = https://github.com/cockroachdb/django-cockroachdb/issues