Add python 3.9 to classifiers in setup.py.

Author: rouilj

setup.py

@@ -151,6 +151,7 @@ def main():
                        'Programming Language :: Python :: 3.6',
                        'Programming Language :: Python :: 3.7',
                        'Programming Language :: Python :: 3.8',
+                       'Programming Language :: Python :: 3.9',
                        'Topic :: Communications :: Email',
                        'Topic :: Office/Business',
                        'Topic :: Software Development :: Bug Tracking',