Add Python 3.9 to the supported versions

Author: seratch

setup.py

@@ -75,6 +75,7 @@
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: Implementation :: CPython",
         "License :: OSI Approved :: MIT License",
         "Operating System :: OS Independent",