added python version 3.9 in setup classifiers

Author: Thibaud

setup.py

@@ -63,6 +63,7 @@
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         "License :: OSI Approved :: Apache Software License",
         "Operating System :: OS Independent",
     ],