setuptools includes a vendored version of six (and other dependencies).
They are not installed through traditional tools. Therefore, distutils
is not required as a fallback to facilitate setuptools.

https://github.com/pypa/setuptools/blob/v40.6.3/setuptools/_vendor/six.py

As documented in the Python 2.6 release notes:

https://docs.python.org/2/whatsnew/2.6.html#pep-3112-byte-literals

> For future compatibility, Python 2.6 adds bytes as a synonym for the
> str type ...

To encourage more forward compatible code bases, inform users of this
builtin alias. This addition is similar in spirit to the note for the
b() function.

Small update to the `ensure_str` function's documentation formatting to
match `ensure_binary` and `ensure_text`'s formatting.

Also, [remove the __sudo__ tag per recommendation of Travis](https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration).

There was `six.io` if and only if py3, which was error-prone.

We are obliged to use a very old version that supports Python 3.2.

This reverts commit d5efa74.

The original intention of providing a distutils fallback has not gone
away. These three extra lines will almost never be used, as most people
have setuptools or install with pip anyway, and at the same time it
serves as a very low-maintenance solution for people who *do* want to
bootstrap setuptools.

Bootstrapping setuptools without vendored six (and pyparsing) is a valid
use case. Although setuptools by default ships with vendored
dependencies, it supports removing the contents of the _vendored/
directory and falling back to an installed version of six.py -- and
thirdparty vendors of setuptools, such as Arch Linux and OpenSUSE,
actually make use of this and need to be able to bootstrap the
dependency tree of setuptools itself.

- Add Python 3.8.
- Unpin pypy2.7 and pypy3.

Fixes #155.
Closes #241.