input: Write a python function to find the prime factors of a given positive number.
  assert prime_factors(30) == [2, 3, 5]
  assert prime_factors(7) == [7]
  assert prime_factors(100) == [2, 2, 5, 5]
  assert prime_factors(84) == [2, 2, 3, 7]
output: def prime_factors(n):
      factors = []
      divisor = 2
      while n >= 2:
          if n % divisor == 0:
              factors.append(divisor)
              n //= divisor
          else:
              divisor += 1
      return factors