Add Python 3.10 classifier

setup.py

@@ -97,6 +97,7 @@ setup(
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: Implementation :: CPython",
         "Programming Language :: Python :: Implementation :: PyPy",
         "Topic :: Office/Business :: Groupware"])