Implement PEP 518 and opt into PEP 517 builds

Merged André requested to merge andre/python-nbxmpp:pep517_518 into master

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

For a summary about, 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
Edited by André

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