setup.py: declare python 3.12 support

setup.py

@@ -289,6 +289,7 @@ setup(
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',
         'Programming Language :: Python :: 3.11',
+        'Programming Language :: Python :: 3.12',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],