Declare typing support via py.typed file

[jwodder]

Running mypy on code that uses the latest version of pypistats fails with the error message "module is installed, but missing library stubs or py.typed marker". A quick look at pypistats's source indicates that it is using type annotations, but because there is no py.typed file, mypy will not use the annotations when type-checking code that uses pypistats.

To resolve this, please do the following:

• If not already, ensure that all public items in pypistats are type-annotated and validated by mypy.
• Add an empty file named py.typed inside src/pypistats/.
• Ensure that the pypistats/py.typed file is included in sdists & wheels built from the project. This may involve configuration specific to your build backend (hatch) that is beyond the scope of this comment.

Further references:

• https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages
• https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker

[hugovk]

Thanks for the suggestion!

I've made a start on it in #434, but likely won't be able to continue this month due to PyCon US and other travel.

Please feel free to take the branch and finish it off :)