Add Python 3.9 to classifiers.

PiperOrigin-RevId: 369482619
Change-Id: I1d9196b4bebb5ed539d2502b2d89aa7df2631a94

Author: mctavish

src/setup.py

@@ -127,6 +127,7 @@ def ReadConfig(section, value, default):
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Development Status :: 3 - Alpha',
         'Intended Audience :: Developers',
     ])