time.sleep calls block pyqt app

pyqt_sevensegment_test.py

#!/usr/bin/env python

"""
PyQt example for seven segment displays.
"""

import sys
import time

from PyQt5 import QtWidgets

import max7219.led as led


class TestApp(object):
    def createGui(self):
        # UI
        self.app = QtWidgets.QApplication(sys.argv)
        self.window = QtWidgets.QMainWindow()

        main = QtWidgets.QWidget()
        layout = QtWidgets.QHBoxLayout()

        self.button = QtWidgets.QPushButton("Start")
        self.slider = QtWidgets.QSlider()
        self.button.clicked.connect(self.toggle)

        layout.addWidget(self.button)
        layout.addWidget(self.slider)
        main.setLayout(layout)

        self.window.setCentralWidget(main)
        self.window.show()
        self.app.exec_()

    def toggle(self):
        if self.button.text() == "Start":
            self.start()
            # note that button text is not updated
            self.button.setText('Stop')
        else:
            self.button.setText('Start')

    def start(self):
        # create seven segment device
        self.device = led.sevensegment(cascaded=1)

        print('Simple text...')
        for _ in range(8):
            self.device.write_text(0, "HELLO")
            time.sleep(0.6)
            self.device.write_text(0, " GOODBYE")
            time.sleep(0.6)

if __name__ == '__main__':
    t = TestApp()
    t.createGui()