Instead of enforcing stringprep or precis, try stringprep and fallback
to precis otherwise.

This makes `python -m nbxmpp.examples.client` work

NOTES: You can set the env variable NBXMPP_USE_PRECIS to revert back
to the old behavior.

Philipp Hörist authored Aug 28, 2022 and Philipp Hörist committed Aug 28, 2022

- Move sending stanzas out of mechanism impl
- Add missing SHA-512-PLUS method
- Add type hints

The default timeout seems to be 60 seconds which is much too long.

Fixes #130

- Use nbxmpp instead of python-nbxmpp
- Unify indentation