graphics/py-python-poppler-qt5: Fix packaging with Python 3.8 and onward

• Add a workaround to make the transition from Python 3.7 to Python 3.8 possible because the name of the .so file is slightly different between the two releases.

• Use PYTHONPREFIX_SITELIBDIR instead PYTHON_SITELIBDIR to make the port PREFIX safe.

• Also add "gl" to USE, introduce USE_{GL,QT} to fix stage QA issues and pet portclippy by sorting the entries of USE_PYQT as well. [1]

PR: 254427
Submitted by: <m.ne@gmx.net> (maintainer) [1]
Approved by: <m.ne@gmx.net> (maintainer)