from tkinter import *
master = Tk()
master.geometry("150x150")
master.title("Новое оконо")
def opn():
mas = Tk()
mas.geometry("250x250")
mas.title("Hello World")
def ext():
exit()
button = Button(master,text="Open new window",command=opn)
btn = Button(master,text="Закрыть окно",command=ext)
button.pack()
btn.pack()
master.mainloop()