configtools.font_is_installed: fix PySide2 compat. issue

[xiaodaxia-2008]

The condition for the follow code should be QT5 instead of PYQT5:

    if PYQT5:
        fontfamilies = QG.QFontDatabase().families()
    else:
        # Qt6
        fontfamilies = QG.QFontDatabase.families()
    return [fam for fam in fontfamilies if str(fam) == font]

[PierreRaybaut]

You are absolutely right.
Thanks for your contribution.