Let tools know this library supports type hints

https://typing.python.org/en/latest/spec/distributing.html#packaging-type-information
https://typing.python.org/en/latest/guides/libraries.html#marking-a-package-as-providing-type-information

It turns out we need to add a py.typed file so that tools will know this
library has type hints. As far as I can tell we don't need to do
anything special to have our build tool (Hatch) include this file when
it builds and packages the library.

Signed-off-by: Nic Cope <nicc@rk0n.org>

pyproject.toml

@@ -14,6 +14,7 @@ classifiers = [
   "Development Status :: 4 - Beta",
   "Programming Language :: Python",
   "Programming Language :: Python :: 3.11",
+  "Typing :: Typed",
 ]
 
 dependencies = [