input: Write a python function to raise each element in a matrix to the power of 3.
  assert power_of_3([[1, 2], [3, 4]]) == [[1, 8], [27, 64]]
  assert power_of_3([[0, -1], [-2, 2]]) == [[0, -1], [-8, 8]]
  assert power_of_3([[2, 3, 4], [5, 6, 7]]) == [[8, 27, 64], [125, 216, 343]]
output: def power_of_3(matrix):
      return [[element ** 3 for element in row] for row in matrix]