input: Write a function to generate a unique 6 digit number.
  assert generate_number()==Some6DigitNumber
  assert generate_number()==SomeOther6DigitNumber
  assert generate_number()==SomeOther6DigitNumber
  assert generate_number()==SomeUnique6DigitNumber
  assert generate_number()==AnotherUnique6DigitNumber
output: import random
  
  def generate_number():
      return random.randint(100000, 999999)