There is a much simpler way to do this, using QDock.toggleViewAction. This function returns a ready-made action that handles the checked state automatically.
So your code would become simply:
self.action = self.dock.toggleViewAction()
self.action.setObjectName("action")
self.menuMenu.addAction(self.action)
self.menubar.addAction(self.menuMenu.menuAction())
self.setWindowTitle("Example of dock remaining minimized")
self.menuMenu.setTitle("Menu")
self.dock.setWindowTitle("I'm a dock")
self.action.setText("Dock visibility")
and you can then get rid of all the signal handling.