Advertise typing status via classifier (#13350)

The package advertises its typing status to typing tools via `py.typed` file.
Adding the Trove Classifier helps advertise its typing status to PyPI browsers.

Without the classifier, this package will not appear in a scoped search:
https://pypi.org/search/?q=mypy&o=&c=Typing+%3A%3A+Typed

setup.py

@@ -179,6 +179,7 @@ def run(self):
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Topic :: Software Development",
+    "Typing :: Typed",
 ]
 
 setup(