Sync classifiers list with tested Python versions

using the list in step_test.yaml

Author: ulgens

pyproject.toml

@@ -22,12 +22,12 @@ classifiers = [
     'Programming Language :: Python',
     'Programming Language :: Python :: 3 :: Only',
     'Programming Language :: Python :: 3',
-    'Programming Language :: Python :: 3.6',
     'Programming Language :: Python :: 3.7',
     'Programming Language :: Python :: 3.8',
     'Programming Language :: Python :: 3.9',
     'Programming Language :: Python :: 3.10',
     'Programming Language :: Python :: 3.11',
+    'Programming Language :: Python :: 3.12',
 ]
 
 dynamic = ["version"]