input: Write a function to get all factors of a number.
  assert get_factors(10) == [1, 2, 5, 10]
  assert get_factors(15) == [1, 3, 5, 15]
  assert get_factors(21) == [1, 3, 7, 21]
output: def get_factors(n):
      factors = []
      for i in range(1, n + 1):
          if n % i == 0:
              factors.append(i)
      return factors