Add py.typed file to mark aioapcaccess as type-annotated (#22)

Per [PEP 561](https://peps.python.org/pep-0561/): "Package maintainers
who wish to support type checking of their code MUST add a marker file
named py.typed to their package supporting typing"

Since `aioapcaccess` is fully typed, this PR adds a `py.typed` file at
the root of the package to support type checking in downstream
dependencies.

Note that we are using `setuptools_scm` plugin such that this file is
automatically included in the final distribution package, without us
having to explicitly specify it in `pyproject.toml`.