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
• Address Use QtPy abstraction layer #144 (review)