input: Write a function to generate a random 6-digit number in Python.
  assert generate_random_number()=='123456'
  assert generate_random_number()=='987654'
  assert generate_random_number()=='555555'
  assert generate_random_number()=='111111'
  assert generate_random_number()=='999999'
output: import random
  
  def generate_random_number():
      return str(random.randint(100000, 999999))