[Python-ideas] Python 4000: Have stringlike objects provide sequence views rather than being sequences

There are many cases in which it is awkward that testing whether an object is a sequence returns `True` for instances of of `str`, `bytes`, etc. This proposal is a serious breakage of backward compatibility, so would be something for Python 4.x, not 3.x. Instead of those objects _being_ sequences, have them provide views that are sequences using a method named something like `members` or `items`.