fix: missing identifier for Python 3.9

Author: henryiii

setup.cfg

@@ -19,6 +19,7 @@ classifiers =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     License :: OSI Approved :: BSD License
     Programming Language :: Python :: Implementation :: PyPy
     Programming Language :: Python :: Implementation :: CPython