Add missing py.typed in module directory

[wdlowry]

Pyright was complaining about a missing stub file. After digging in a little, I noticed that py.typed is missing the module directory. This PR adds it.

Test:

❯ python -m build
...
Successfully built mailgun-1.6.0.post1.dev1.tar.gz and mailgun-1.6.0.post1.dev1-py3-none-any.whl
❯ unzip -l dist/mailgun-1.6.0.post1.dev1-py3-none-any.whl | grep py.typed
        0  02-12-2026 13:23   mailgun/py.typed