input: Write a function to generate a random 6-digit number in Python. The function should not take any input and should return a string representation of a random 6-digit number.
  assert generate_6_digit_number()==str(random.randint(100000, 999999))
  assert len(generate_6_digit_number())==6
  assert generate_6_digit_number().isdigit()==True
output: import random
  
  def generate_6_digit_number():
      return str(random.randint(100000, 999999))