sysutils/py-crontab: Rename to python-crontab, Update to 2.4.0

• Rename the port to match the canonical/registered (PyPI/CHEESESHOP) name
• Switch from CHEESESHOP to GitLab for complete tests data
• Update the project website URL in pkg-descr
• Add LICENSE_FILE
• Add sysutils/py-cron-descriptor to RUN_DEPENDS
• Add test target

Reviewed by: koobs
Approved by: koobs (ports, mentor), portmgr (maintainer timeout: > 2 weeks)
Differential Revision: https://reviews.freebsd.org/D23968