title: Not running class destructor when i close app in python and pyqt
body: <p>When I get out of program(sys.exit(app.exec_())),Main Form closes, but there are two problems:<br>
  1 - Destructor of MainForm class not run<br>
  2 - Thread is still running too<br>
  I want when i close app, destructor of MainForm is runed and all threads are killed too</p>
  
  <pre><code>    class MainForm(QMainWindow,Ui_MainWindow):
          def __init__(self,parent=None):
              super(MainForm,self).__init__(parent)
              self.setupUi(self)
              #...
          def init_main_form(self):
              #...
              self.show_time()
          def show_time(self):
              self.label_9.setText(u"{}:{}:{}".format(str(datetime.datetime.now().hour),str(datetime.datetime.now().minute),str(datetime.datetime.now().second)))
              self.label_9.resize(self.label_9.width()+len(self.label_9.text())*3,self.label_9.height())
              b = threading.Timer(1,self.show_time)
              #b.setName('localtime')
              #self.thread_list.append(b)
              b.start()
          def __del__(self):
              print("app is closed")
              for tr in threading.enumerate():
                  if tr.isAlive():
                      tr._Thread__stop()
                      # or tr.finished
                      # or tr.terminate()
      def main():
          app = QApplication(sys.argv)
          main_form = MainForm()
          main_form.show()
          sys.exit(app.exec_())
  
      if __name__ == '__main__':
          main()
  </code></pre>