Goes to the previous page for this wizard.

def next(self):
        """
        Goes to the previous page for this wizard.
        """
        curr_page = self.currentPage()
        if not curr_page:
            return
        elif not curr_page.validatePage():
            return

        pageId = curr_page.nextId()
        try:
            next_page = self._pages[pageId]
        except KeyError:
            return

        self._currentId = pageId
        self._navigation.append(pageId)

        y = curr_page.y()
        next_page.move(self.width(), y)

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

        # animate the current page out
        anim_out = QtCore.QPropertyAnimation(self)
        anim_out.setTargetObject(next_page)
        anim_out.setPropertyName('pos')
        anim_out.setStartValue(next_page.pos())
        anim_out.setEndValue(curr_page.pos())
        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)

        next_page.show()

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

        # initialize the new page
        self.currentIdChanged.emit(pageId)
        next_page.initializePage()
        anim_grp.start()