Port Qiskit Metal from PySide2 to PySide6

[obrienpja]

What are the issues this pull addresses (issue numbers / links)?

Porting Qiskit Metal from PySide2 to PySide6

Did you add tests to cover your changes (yes/no)?

No, but I don't think we have any tests for the GUI itself anyway

Did you update the documentation accordingly (yes/no)?

No, none of the examples should be affected

Did you read the CONTRIBUTING document (yes/no)?

Yes

Summary

The commits change all of the PySide2 imports to PySide6 imports. It also changes the matplotlib backend from Qt5Agg to QtAgg.

Details and comments

A couple of classes were moved in PySide6. For example, QAction moved from QtWidgets in PySide2 to QtGui in PySide6. We also needed to change the backend that handles the rendering of matplotlib from Qt5Agg to QtAgg. According to their documentation, the QtAgg backend should be able to handle the connection between any version of Qt and matplotlib. From what I remember, they are doing their best to make it agnostic.

[obrienpja]

When this is merged, it will resolve #798.

[CLAassistant]

All committers have signed the CLA.