Fix for #489669 (Neil Norwitz): memory leak in test_descr (unicode).

Fix for #489669 (Neil Norwitz): memory leak in test_descr (unicode).

This is best reproduced by

  while 1:
      class U(unicode):
          pass
      U(u"xxxxxx")

The unicode_dealloc() code wasn't properly freeing the str and defenc
fields of the Unicode object when freeing a subtype instance.  Fixed
this by a subtle refactoring that actually reduces the amount of code
slightly.