title: "QStackedWidget is not defined" when using PyQt5 on Python 3.8
body: <p>It's my very first time trying to create a GUI with Python, so I decided to try PyQt5. I followed a tutorial, but I'm being stuck on a weird error which I couldn't find anywhere online : <strong>QStackedWidget is not defined</strong>.</p>
  <p>My code is pretty simplistic for now, and I believe I have imported each module from the PyQt5 library I need.</p>
  <pre><code>from PyQt5.uic import loadUi
  from PyQt5 import QtWidgets
  from PyQt5.QtWidgets import QApplication, QDialog, QWidget
  import sys
  
  class mainMenu(QDialog):
      def __init__(self):
          super(mainMenu, self).__init__()
          loadUi(&quot;MainMenu.ui&quot;, self)
  
  app = QApplication(sys.argv)
  menu = mainMenu()
  widget = QStackedWidget()
  widget.addWidget(menu)
  widget.setFixedHeight(800)
  widget.setFixedWidth(1200)
  widget.show()
  try:
      sys.exit(app_exec())
  except:
      print(&quot;Exiting&quot;)
  </code></pre>
  <p>So yeah, I'm not sure what the problem is... Maybe I f*cked up my PyQt5 install?</p>