celery: only accept msgpack content

• SECURITY Forces Celery to only accept msgpack content when using standard configuration. This disallows pickle messages which can be used for remote code execution. (closes #3003)

• INCOMPATIBILITY If you use any Celery serializer other than msgpack, you must update configuration variable CELERY_ACCEPT_CONTENT to include that serializer.

• Adds celery to modules recognized by kwalitee

Signed-off-by: Marco Neumann <marco@crepererum.net>