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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

PRIORITY_DEFAULT is used for example for X Server events or other UI
tasks. This should help to prefer UI events before network events.

Returns True if the stream is resumeable via stream management

This functionality is still desired

Its unspecified if servers need to send a full JID or bare JID here.
Further its also unspecified if they need to send all full JIDs of
all connected resources.

So we implement on what we can actually depend, there needs to be
at least a bare JID.