Advertise Python 3.9 support

Change-Id: If6bd1dc7e6d5eb0c090387af7d8234b586c50137
diff --git a/setup.py b/setup.py
index 29dfb1d..adeea90 100644
--- a/setup.py
+++ b/setup.py
@@ -19,6 +19,7 @@
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Intended Audience :: Developers',