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