devel/py-hologram: new port

Hologram is a lightweight library to generate Draft 7 compatible JSON Schemas
from python 3.7 dataclasses. Python 3.6 is supported through the dataclasses
backport. It is forked from dataclasses-jsonschema created by s-knibbs.

PR: 274579
Author: Pat Maddox <pat@patmaddox.com>