input: Write a function to generate a unique 6-digit number. The function should return a random 6-digit number.
  assert generate_number() != generate_number()
  assert len(str(generate_number())) == 6
  assert str(generate_number()).isdigit() == True
  assert generate_number() >= 100000 and generate_number() <= 999999
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)