Add Python 3.9 as supported

Author: nabla-c0d3

setup.py

@@ -41,6 +41,7 @@
         "License :: OSI Approved :: GNU Affero General Public License v3",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Topic :: System :: Networking",
         "Topic :: System :: Monitoring",
         "Topic :: System :: Networking :: Monitoring",