Previously _output_buf was racy. This may or may not be cheaper than
simply using a lock, but it requires much less code, so I prefer it for
now.

Defining functions in the current module was causing the entirety of
py.test and all dependencies to be sucked in.

Original commit:

commit 3535e45245f69b11fe4c9d40f5b07b4c068b0ac4
Author: David Wilson <dw@botanicus.net>
Date:   Fri Aug 26 19:35:09 2016 +0100

    Broken SSH unit test.

A  todo/six-module-import-error.py