Add ABC ByteString which unifies bytes and bytearray (but not memoryview).

Add ABC ByteString which unifies bytes and bytearray (but not memoryview).
There's no ABC for "PEP 3118 style buffer API objects" because there's no
way to recognize these in Python (apart from trying to use memoryview()
on them).
Note that array.array really should be registered as a MutableSequence
but that would require importing it whenever collections is imported.