devel/ghpr: update to v3

v3 makes the following high-level changes apart from the standard
version update described in [1]:

• pyproject.toml / PEP-517 using setuptools_scm integration.
• Now the package is prefixed with PYTHON_PKGNAMEPREFIX so multiple versions of the package can be built with different versions of python and installed concurrently (if one decided to do that).
• LICENSE.md from the latest version is used/incorporated into the end-resulting package.
• Make the port portlint clean by reordering variables and using PORTVERSION instead of DISTVERSION.

Approved by: imp (maintainer)
Differential Revision: https://reviews.freebsd.org/D55892