What you'd want to do is create different frames, each with their own positioning logic. Take a look at this:
import tkinter
root = tkinter.Tk()
frame = tkinter.Frame(root)
frame.pack()
bottomframe = tkinter.Frame(root)
bottomframe.pack(side=tkinter.BOTTOM)
tkinter.Button(frame, text="left").pack(side=tkinter.LEFT)
tkinter.Button(frame, text="middle").pack(side=tkinter.LEFT)
tkinter.Button(frame, text="right").pack(side=tkinter.LEFT)
# what you want
tkinter.Button(bottomframe, text="top").pack(side=tkinter.TOP)
tkinter.Button(bottomframe, text="under").pack(side=tkinter.TOP)
root.mainloop()