setup.py: adding python 3.9.

Author: mrjbq7

setup.py

@@ -159,6 +159,7 @@ def build_extensions(self):
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Cython",
         "Topic :: Office/Business :: Financial",
         "Topic :: Scientific/Engineering :: Mathematics",