setup.cfg: Extend Python 3 classifiers

Author: hartwork

setup.cfg

@@ -15,6 +15,10 @@ classifier =
     Operating System :: OS Independent
     Programming Language :: Python
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
     Framework :: Django
 
 [options]