Add back 3.6 classifier

Author: oschwald

setup.py

@@ -123,6 +123,7 @@ def run_setup(with_cext):
             "Intended Audience :: System Administrators",
             "License :: OSI Approved :: Apache Software License",
             "Programming Language :: Python :: 3",
+            "Programming Language :: Python :: 3.6",
             "Programming Language :: Python :: 3.7",
             "Programming Language :: Python :: 3.8",
             "Programming Language :: Python",