Fixes #2497

QMenus are instantiated and then added to another menu as a sub-menu using QMenu.addMenu(menu : QMenu).
However, this method specifically does not take ownership of the sub-menu. (https://doc.qt.io/qt-6/qmenu.html#addMenu)
Hence the sub-menus remain parent-less.

On PySide{2,6} bindings, these parent-less sub-menus get leaked, as demonstrated by the scripts in #2497.
Setting a parent to them fixes the leak.

However, these changes cause a failure in test_ref_cycles.py::test_PlotWidget() for PyQt{5,6} bindings.
Specifically, with this change, the objects in w.plotItem.getMenu() already have their underlying C++ object destroyed, resulting in the following error.

>           assert ref() is None
E           AssertionError: assert <PyQt6.QtWidgets.QCheckBox object at 0x000001BC7D7EC310> is None
E            +  where <PyQt6.QtWidgets.QCheckBox object at 0x000001BC7D7EC310> = <[RuntimeError('wrapped C/C++ object of type QCheckBox has been deleted') raised in repr()] weakref object at 0x1bc7d804c20>()

A possible workaround (done in this PR) is to treat such occurrences as a pass.