input: Write a function to find the prime factors of a given number.
  assert prime_factors(28) == [2, 2, 7]
  assert prime_factors(45) == [3, 3, 5]
  assert prime_factors(13) == [13]
  assert prime_factors(100) == [2, 2, 5, 5]
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