devel/py-pygobject: rename and update to 3.50.0

The proper upstream Python package name is pygobject; version 2
hasn't existed or been supported in ages, so rename.

Rework the port to where -common is the child port but the main
port depends on it for the header files. Build the main port as a
Python wheel USE_PYTHON=pep517 to take advantage of USE_PYTHON=autoplist.

Direct -common consumers have PORTREVISION bumped.

Changelog: https://gitlab.gnome.org/GNOME/pygobject/-/blob/3.50.0/NEWS

Reported by: Olivier Duchateau <duchateau.olivier[at]gmail[dot]com> (3.44, 3.46)
Exp-run by: antoine
PR: 273722
Differential Revision: https://reviews.freebsd.org/D47635