devel/py-celery: Add missing dependency to devel/py-python-dateutil

The package is required: https://github.com/celery/celery/blob/bf1c98baba7431ee6a60d5972c5759699473acad/requirements/default.txt#L9

Approved by: jrm (mentor), olgeni (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D51327