input: Write a python function to calculate the slope of a line given two points.
  assert slope((1, 0), (0, 1)) == -1.0
  assert slope((2, 3), (2, 7)) == None
  assert slope((1, 2), (3, 6)) == 2.0
output: def slope(point1, point2):
      x1, y1 = point1
      x2, y2 = point2
      if x1 == x2:
          return None
      return (y2 - y1) / (x2 - x1)