[docs] [issue26696] Document collections.abc.ByteString

[Brett Cannon]

Changes by Brett Cannon <brett at python.org>:


----------
assignee:  -> docs at python
components: +Documentation -Library (Lib)
nosy: +docs at python

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26696>
_______________________________________