input: Write a function to create an n x n matrix filled with random integers between 1 and 9.
  assert len(fill_matrix(3)) == 3 and all(len(row) == 3 for row in fill_matrix(3))
  assert len(fill_matrix(5)) == 5 and all(len(row) == 5 for row in fill_matrix(5))
  assert all(1 <= cell <= 9 for row in fill_matrix(4) for cell in row)
output: import random
  
  def fill_matrix(n):
      return [[random.randint(1, 9) for _ in range(n)] for _ in range(n)]