code: def from_birth_date(birth_date):
          
          if birth_date > datetime.date.today():
              raise ValueError('birth_date can\'t be in the future')
          date_range = DateRange(birth_date, birth_date)
          return DOB(date_range=date_range)
docs: Take a person's birth date (datetime.date) and return a new DOB 
          object suitable for him.