Update python version trove classifiers

This should work in all current-ish versions.

Change-Id: Ic97fcfc554fe7dd5cfa5a9298a4b13892d1c97f8

setup.cfg

@@ -16,6 +16,10 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 keywords = gerrit console urwid review
 
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