Add py.typed file to mark aioapcaccess as type-annotated

[yuxincs]

Per PEP 561: "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.