Use QtPy abstraction layer (#144). GHA tests with PySide2 and PyQt5

[paskino]

Builds on #144 and installs Qt bindings in GHA tests.
From @samtygier-stfc :

This switches to importing Qt classes via the qtpy abstraction layer. The library will then use which ever Qt binding you have in your environment. If you have multiple, its possible to select one using an environment variable (see https://github.com/spyder-ide/qtpy).

A small change to some mocking in a test was needed.

This passes all the tests, but has not been tested for an application that uses the library.

An application that uses eqt, will now need to add a dependency on the Qt binding that it wants to use.

Because now an app installing eqt will not have installed a Qt binding by default this is a backward incompatible change and will require a major version change.

• closes Enable changing of Qt backend #2
• supersedes Use QtPy abstraction layer #144
• GHA install and test PySide2 and PyQt5 Qt bindings
• Manual test on an app (see comment by @DanicaSTFC below)
• Address Use QtPy abstraction layer #144 (review) i.e. "The user will need to remember to install a Qt binding package. Do we want that?" (see comment by @DanicaSTFC below)
• Fix the conda-forge feedstock https://github.com/conda-forge/eqt-feedstock/blob/main/recipe/meta.yaml#L25

[DanicaSTFC]

• Merge main from 10/12/2024
• Run examples in eqt with pyside2.
• Run examples in eqt with qtpy5. Opened issue Reordable_list_widget_example does not run with qtpy5 #167 for one example not working well.
• Fix example "Dialog_save_state_example"
• Test that everything works in "iDVC" repository
• Test that everything works in "CILviewer" repository
• Add readme to explain that the user must have pyside2 or pyqt5 in readme file.
• add 2 yml files so that the user can create the environment that they need, with either pyside2 or qtpy5. There will be one common file for the generic environment.
• Tested that the new env files work well by running the examples
• Update the meta in conda-forge https://github.com/conda-forge/eqt-feedstock/tree/main/recipe. Opened PR https://github.com/conda-forge/eqt-feedstock/pull/5/files
• Update changelog
• Fix failing unit tests by removing the appearance of QMenu and findChildren.

=================================================================== FAILURES ====================================================================
______________________________ TestMainWindowWithSessionManagementMenuBar.test_createMenu_sets_menu_bar_and_menus _______________________________
test\test_MainWindowWithSessionManagement.py:105: in test_createMenu_sets_menu_bar_and_menus
    assert isinstance(self.smw.menus["File"], QMenu)
E   assert False
E    +  where False = isinstance(<PySide2.QtWidgets.QMenu(0x1a5fdea1460) at 0x000001A5F984BB00>, QMenu)
--------------------------------------------------------------- Captured log call ---------------------------------------------------------------
INFO     qdarkstyle:__init__.py:269 QSS file successfully loaded.
INFO     qdarkstyle:__init__.py:172 Found version patches to be applied.
INFO     qdarkstyle:__init__.py:192 Found application patches to be applied.
________________________________ TestMainWindowWithSessionManagementMenuBar.test_file_menu_has_expected_actions _________________________________
test\test_MainWindowWithSessionManagement.py:115: in test_file_menu_has_expected_actions
    file_menu = menus[0]
E   IndexError: list index out of range
--------------------------------------------------------------- Captured log call ---------------------------------------------------------------
INFO     qdarkstyle:__init__.py:269 QSS file successfully loaded.
INFO     qdarkstyle:__init__.py:172 Found version patches to be applied.
INFO     qdarkstyle:__init__.py:192 Found application patches to be applied.
______________________________ TestMainWindowWithSessionManagementMenuBar.test_settings_menu_has_expected_actions _______________________________
test\test_MainWindowWithSessionManagement.py:122: in test_settings_menu_has_expected_actions
    settings_menu = menus[1]
E   IndexError: list index out of range

QMenu type is <class 'Shiboken.ObjectType'>