input: Write a python function to generate a square matrix of given size with random integers between 0 and 9.
  assert len(matrix(3))==3 and all(len(row)==3 for row in matrix(3))
  assert len(matrix(5))==5 and all(len(row)==5 for row in matrix(5))
  assert len(matrix(1))==1 and all(len(row)==1 for row in matrix(1))
output: import random
  
  def matrix(size):
      return [[random.randint(0, 9) for _ in range(size)] for _ in range(size)]