Starting from Qt6.0, classes QOpenGLXXXX (such as QOpenGLBuffer) have been migrated to the QtOpenGL module.
Therefore, the
<object type name="QOpenGLBuffer"/>
in typesystem_gui.xml should be changed to
<object type name="QOpenGLBuffer" before-version="6"/>,
and then
<object type name="QOpenGLBuffer" since-version="6"/>
should be added to typesystem_opengl.xml. Other QOpenGLXXXXs should also be treated similarly,
while the original QGLBuffer in typesystem_opengl.xml should be changed to
<object type name="QGLBuffer Before-version=" 6 "/>

int PythonQt_QtAll.pro

  lessThan(QT_MAJOR_VERSION, 6) {
    # module is empty in Qt6
    qtHaveModule(opengl):CONFIG += PythonQtOpengl
  }

changto:
qtHaveModule(opengl):CONFIG += PythonQtOpengl

PythonQtOpengl {
  DEFINES += PYTHONQT_WITH_OPENGL
  QT += opengl
  PythonQtCore: Xinclude (com_trolltech_qt_opengl)
  QT += xml
}

changto:

PythonQtOpengl {
  DEFINES += PYTHONQT_WITH_OPENGL
  QT += opengl
  equals(QT_MAJOR_VERSION, 6){
    QT += openglwidgets
  }
  PythonQtCore: Xinclude (com_trolltech_qt_opengl)
  QT += xml
}