input: Write a function to find the prime factors of a given number.
  assert prime_factors(315)==[3, 3, 5, 7]
  assert prime_factors(84)==[2, 2, 3, 7]
  assert prime_factors(29)==[29]
output: def prime_factors(n):
      factors = []
      divisor = 2
      while n >= 2:
          while n % divisor == 0:
              factors.append(divisor)
              n //= divisor
          divisor += 1
      return factors