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.

--- a/development.ini	Mon Feb 24 18:37:31 2020 +0100
+++ b/development.ini	Thu Feb 20 02:03:24 2020 +0100
@@ -250,6 +250,7 @@
 ###        CELERY CONFIG        ####
 ####################################
 
+## Note: Celery doesn't support Windows.
 use_celery = false
 
 ## Example: use the message queue on the local virtual host 'kallitheavhost' as the RabbitMQ user 'kallithea':