Make package PEP561 compatible

[smarie]

See https://mypy.readthedocs.io/en/latest/installed_packages.html#making-pep-561-compatible-packages

We should add a py.typed and the following to setup.py :

    package_data={"": ['py.typed', '*.pyi']},

    # explicitly setting the flag to avoid `ply` being downloaded
    # see https://github.com/smarie/python-getversion/pull/5
    # and to make mypy happy
    # see https://mypy.readthedocs.io/en/latest/installed_packages.html
    zip_safe=False,