I think a warning at the top of StringIO docs is needed.

And it should link to io.BytesIO.

Maybe even deprecate StringIO and cStringIO in Python2?

StringIO docs: https://docs.python.org/2/library/stringio.html

io.BytesIO docs: https://docs.python.org/2/library/io.html#io.BytesIO

I would like to see this at the top of StringIO:

{{{
Please use io.BytesIO and io.StringIO since this module is not supported in Python3
}}

Python 2 is EOL.