Adding Python 3.9 classifier to setup.py

Author: dantownsend

setup.py

@@ -49,6 +49,7 @@
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: Implementation :: CPython",
         "Framework :: AsyncIO",
         "Typing :: Typed",