Fixes #126

The support of setup.py is deprecated by setuptools and will be removed in the future.

For a summary about setup.py, PEP 517 [1] and 518 [2], see [3].

To install nbxmpp (still) run

   pip install .

To package nbxmpp use

    pip install build
    python -m build

[1]: https://peps.python.org/pep-0517/
[2]: https://peps.python.org/pep-0518/
[3]: https://lists.archlinux.org/pipermail/arch-dev-public/2022-February/030737.html

To install nbxmpp together with the optional dependeny run
`pip install .[gssapi]`.

- Correct the default value for subscription