Command now takes stdout, stderr, and stdin arguments. These are

expected, in the case of custom streams, to be io.TextIOWrapper

instances, wrapping a io.BytesIO.
The standard and JSON output wrappers on Command instances will wrap

these instead of assuming the versions on sys. The log stream will

wrap stderr as well (it normally defaults to sys.stderr), ensuring

that all logs are, by default, outputted to that stream. This can, as

always, be overridden with a more explicit logging setup if automating

RBTools.
There's also *_is_atty attributes computed for each of these streams

when instantiating. These replace the previous .isatty() checks, and

are more robust, handling the case where this attribute is missing.
This will be used for unit tests that need to capture command output or

simulate input.

Tested running RBTools commands on Python 2, 3.6, and 3.10. Checked

default stdout, stderr, and stdin functionality.
Made use of this in upcoming unit test support. Verified that I could

override the streams successfully on all supported versions of Python

and grab the resulting output (or supply the resulting input).