Features
* `stdio_mgr` context manager with capability for mocking/wrapping all three
of `stdin`/`stdout`/`stderr`
* `stdin` mocking/wrapping is implemented with the custom `TeeStdin`, a
subclass of `StringIO`, which tees all content read from itself into
the mocked/wrapped `stdout`
* `TeeStdin` is extended from `StringIO` by an `.append()` method,
which adds content to the end of the stream without changing the
current seek position.