input: Write a function to count the number of divisors of a given number.
  assert count_divisors(10) == 4
  assert count_divisors(15) == 4
  assert count_divisors(21) == 4
output: def count_divisors(n):
      count = 0
      for i in range(1, n + 1):
          if n % i == 0:
              count += 1
      return count