The ujson module apparently will accept both bytes and text format
input, however, it does always output a str (both on Python 2 and
Python 3).
Some discussion in: https://github.com/python/typeshed/pull/460
* subprocess.CalledProcessError output argument is optional.
* subprocess.CalledProcessError takes stderr argument in python3.
* subprocess: Fix type for command in call() and friends.
The stubs didn't correctly support the fact that you can pass a string
as well as a sequence of strings.
