Goes to the previous page for this wizard.

def back(self):
        """
        Goes to the previous page for this wizard.
        """
        try:
            pageId = self._navigation[-2]
            last_page = self.page(pageId)
        except IndexError:
            return

        curr_page = self.page(self._navigation.pop())
        if not (last_page and curr_page):
            return

        self._currentId = pageId

        y = curr_page.y()
        last_page.move(-last_page.width(), y)
        last_page.show()

        # animate the last page in
        anim_in = QtCore.QPropertyAnimation(self)
        anim_in.setTargetObject(last_page)
        anim_in.setPropertyName('pos')
        anim_in.setStartValue(last_page.pos())
        anim_in.setEndValue(curr_page.pos())
        anim_in.setDuration(self.animationSpeed())
        anim_in.setEasingCurve(QtCore.QEasingCurve.Linear)

        # animate the current page out
        anim_out = QtCore.QPropertyAnimation(self)
        anim_out.setTargetObject(curr_page)
        anim_out.setPropertyName('pos')
        anim_out.setStartValue(curr_page.pos())
        anim_out.setEndValue(QtCore.QPoint(self.width()+curr_page.width(), y))
        anim_out.setDuration(self.animationSpeed())
        anim_out.setEasingCurve(QtCore.QEasingCurve.Linear)

        # create the anim group
        anim_grp = QtCore.QParallelAnimationGroup(self)
        anim_grp.addAnimation(anim_in)
        anim_grp.addAnimation(anim_out)
        anim_grp.finished.connect(curr_page.hide)
        anim_grp.finished.connect(anim_grp.deleteLater)

        # update the button states
        self._buttons[self.WizardButton.BackButton].setVisible(self.canGoBack())
        self._buttons[self.WizardButton.NextButton].setVisible(True)
        self._buttons[self.WizardButton.RetryButton].setVisible(self.canRetry())
        self._buttons[self.WizardButton.CommitButton].setVisible(last_page.isCommitPage())
        self._buttons[self.WizardButton.FinishButton].setVisible(last_page.isFinalPage())

        self.adjustSize()
        self.currentIdChanged.emit(pageId)
        anim_grp.start()