add python 3.6 classifier

Author: joeljonsson

setup.cfg

@@ -12,6 +12,7 @@ classifiers =
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Programming Language :: Python ::
+    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9