PyQt5: attach button position to the window coordinates

So, I'm trying to experiment a bit with PyQt5. I have created a basic window with two QPushButtons.

from PyQt5 import QtWidgets
from PyQt5.QtWidgets import QApplication, QMainWindow, QSizePolicy
import sys

class Okno(QMainWindow):
    def __init__(self):
        super(Okno, self).__init__()
        self.setGeometry(500, 500, 900, 500)  # window size setGeometry(xpos, ypos, w, h)
        # self.setFixedSize(900, 500) # fix window position
        self.setWindowTitle("My window")
        self.label = QtWidgets.QLabel(self)
        self.button1 = QtWidgets.QPushButton(self)
        self.button2 = QtWidgets.QPushButton(self)
        self.iniUI()

    # Buttons
    def iniUI(self):
        self.button1.setText("Open file")
        self.button1.move(75, 450)
        self.button1.setMinimumWidth(150)
        self.button1.clicked.connect(Button_events.open_file)  # add (connect) button event 

        self.button2.setText("Exit")
        self.button2.move(750, 450)
        self.button2.clicked.connect(exit)

# Button events
class Button_events(QMainWindow):
    def open_file(self):
        print("Open file")

# Main method
def window():
    app = QApplication(sys.argv)
    okno = Okno()
    okno.show()
    sys.exit(app.exec_())


window()

I have set button's position by absolute coordinates. When I resize My Window, the button simply holds its absolute coordinates.

How can I attach the button's coordinates to My Window, so when I scale the window, the button would move within.

Is it possible to set a minimum My Window size with respect to minimum QPushButtons size (when buttons won't fit My Window, it can't be adjusted any smaller)?

Here is a simple screen of what I'm trying to achieve (the original buttons are cut in half, they would disappear if My Window gets any smaller):

Thank you.