code: def from_birth_year(birth_year):
          
          if birth_year <= 0:
              raise ValueError('birth_year must be positive')
          date_range = DateRange.from_years_range(birth_year, birth_year)
          return DOB(date_range=date_range)
docs: Take a person's birth year (int) and return a new DOB object 
          suitable for him.