celery: upgrade to Celery 4 Celery 3 doesn't support Python 3.7 or later. This upgrade is thus essential for full Python 3 support. But note that https://docs.celeryproject.org/en/4.4.0/faq.html#does-celery-support-windows says "No". The names of config settings changed in Celery 3 to 4, as described on https://docs.celeryproject.org/en/3.0/whatsnew-4.0.html#lowercase-setting-names . Celery 4 config settings can now be specified in Kallithea .ini files by prefixing with `celery.` - for example as `celery.broker_url`. Remain backwards compatible for the usual settings, and map old names to the new names.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.