[NEW PORT] sysutils/py-cron-descriptor: Converts cron expressions into human readable strings

A Python library that converts cron expressions into human
readable strings. Ported to Python from
https://github.com/bradyholt/cron-expression-descriptor.

WWW: https://github.com/Salamek/cron-descriptor

Reviewed by: koobs
Approved by: koobs (ports)
Differential Revision: https://reviews.freebsd.org/D23018