How to adjust the geometry of widgets added in vertical box layout in pyqt5

I have created a pyqt5 window which has a vertical layout. Inside this vertical layout, I have added 2 buttons. By default these are vertically aligned like below:

How can I adjust the geometry of the buttons to move above. Expected output like below:

so that if I add 3rd button, it goes below button 2. Below is the code:

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QWidget, QPushButton


class MainWindow(QMainWindow):

    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)

        layout = QVBoxLayout()
        widget = QWidget()
        widget.setLayout(layout)
        self.setCentralWidget(widget)

        button1 = QPushButton('Button 1', self)
        layout.addWidget(button1)

        button2 = QPushButton('Button 2', self)
        layout.addWidget(button2)


app = QApplication(sys.argv)
main_window = MainWindow()
main_window.show()
sys.exit(app.exec_())