Ship `py.typed` file for `mypy` support

[adamtheturtle]

Is your feature request related to a problem? Please describe.

When I use mypy and piq, I get an error like the following:

mypy example.py
example.py:1: error: Skipping analyzing "piq": module is installed, but missing library stubs or py.typed marker  [import-untyped]
example.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

Even if I # type: ignore the error, I do not get the benefits of type safety when using piq.
Similar errors occur with other type checkers (pyre, pyright).

See https://peps.python.org/pep-0561/#packaging-type-information.

Describe the solution you'd like

Add an empty py.typed file to piq. This tells mypy to use the types defined in piq.

Describe alternatives you've considered

• Use stubgen to create a types-piq package.