Update programming language mentioned in setup.cfg

As py35 has been dropped and py36, py37 jobs are running, so
updated the setup.cfg accordingly.

Change-Id: I09eae818f3d4188444aba8f1ece9d3d11eda95c2

setup.cfg

@@ -17,7 +17,8 @@ classifier =
     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
 
 [files]
 packages =