input: Write a function to find the prime factors of a given number.
  assert prime_factorize(10) == [2, 5]
  assert prime_factorize(15) == [3, 5]
  assert prime_factorize(21) == [3, 7]
  assert prime_factorize(1) == []
  assert prime_factorize(2) == [2]
output: def prime_factorize(n):
      if n <= 1:
          return []
      factors = []
      divisor = 2
      while n >= divisor ** 2:
          if n % divisor == 0:
              factors.append(divisor)
              n //= divisor
          else:
              divisor += 1
      if n > 1:
          factors.append(n)
      return factors