Dropdown menu on a toolbar button don't show up in PyQt

Question

I do use PyQt5 and try to create a toolbar where one of the buttons do have a drop down menu with two entries.

The code below works without exceptions. The button do have its dropdown arrow right of it. But clicking on that arrow has no effect. The menu do not pop up.

#!/usr/bin/env python3
import sys
from PyQt5.QtWidgets import (
    QMainWindow, QApplication, QAction, QStyle, QToolButton, QMenu)


class Window(QMainWindow):
    def __init__(self, parent=None):
        """Initializer."""
        super().__init__(parent)

        # Actions
        ico_one = self.style().standardIcon(QStyle.SP_DirIcon)
        self.act_one = QAction(ico_one, 'one', self)
        self.act_one.triggered.connect(lambda: print('one'))

        ico_sub_one = self.style().standardIcon(QStyle.SP_ArrowBack)
        self.act_sub_one = QAction(ico_sub_one, 'sub one', self)
        self.act_sub_one.triggered.connect(lambda: print('sub-one'))

        ico_two = self.style().standardIcon(QStyle.SP_TrashIcon)
        self.act_two = QAction(ico_two, 'two', self)
        self.act_two.triggered.connect(lambda: print('two'))

        # Toolbar
        toolbar = self.addToolBar('main')

        # Add actions to toolbar
        toolbar.addAction(self.act_one)
        toolbar.addAction(self.act_two)

        # create sub-menu for one toolbarbutton
        wdg_menu = QMenu()
        wdg_menu.addAction(self.act_one)
        wdg_menu.addAction(self.act_sub_one)

        # get toolbar button
        wdg = toolbar.widgetForAction(self.act_one)

        # add sub-menu to it
        wdg.setMenu(wdg_menu)
        wdg.setPopupMode(QToolButton.MenuButtonPopup)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    win = Window()
    win.show()
    sys.exit(app.exec_())