Adding exec method to QtCompat

[Jerakin]

Adding exec_ as a QtCompat. While it isn't removed in PySide6 it is currently emitting a Deprecation warning. I am unsure when it is slated for removal.

D:\...\__main__.py:61: DeprecationWarning: 'exec_' will be removed in the future. Use 'exec' instead.
  sys.exit(app.exec_())

Current usage.

app = QtWidgets.QApplication(sys.argv)
window = Window()
window.show()
sys.exit(app.exec_())

Suggested usage with QtCompat

app = QtWidgets.QApplication(sys.argv)
window = Window()
window.show()
sys.exit(QtCompat.exec_(app))

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[Jerakin]

Local branch didn't have the PySide6 changes. This has already been resolved.