If the app-iterator being passed in is not defining `next()` (e.g. running under py3 and only defined `__next__`), this will currently crash.

Fix by using `six.next` which will use `__next__` or `next` as appropriate.

* add ipv6 support by setting address family

* StringIO imports differently in python 3 - use six for compatibility

* Paste requires bytes like objects.  in python 2 cStringIO works fine, but in python three it needs to use io.BytesIO

* simplify imports and logic by using six.BytesIO exclusively for parsing body content

`pytest-runner` describes itself as deprecated in its own package
description, referring to
https://github.com/pypa/setuptools/issues/1684.  Using it in
`setup_requires` means that any other package that depends on Paste has
to install `pytest-runner` too, which is fairly heavyweight and
unnecessary.  (I ran into this when trying to update Launchpad to a less
ancient version of Paste.)

This does mean that `python setup.py test` no longer works, but,
according to the setuptools issue above, that's deprecated anyway.
Paste already has `tox` configuration that works well.

more python3 fixes