Change the qt calls to use qtpy to deal with different Qt bindings

Today the best approach to ensure some consistency and flexibility across different Qt python bindings (PyQt5, PySide2, PyQt6, PySide6, ...) is the use of [QtPy: Abstraction layer for PyQt5/PyQt4/PySide2/PySide](https://github.com/spyder-ide/qtpy).

I propose a set of minor changes to ensure all the calls to Qt bindings are performed via from qtpy import.

By default the module to be called will be PyQt5 but it can be changed by setting the environment variable QT_API.

If the proposal is accepted, I will submit a git commit against the latest version.