Use BytesIO instead of StringIO

[radarhere]

Updates the tutorial in light of #4302, and in light of the fact that StringIO.StringIO doesn't exist in Python 3.

[hugovk]

The failures are an unrelated problem in the newly-released pip 20.0.0: pypa/pip#7620

I expect there'll be a pip 20.0.1 soonish, shall we do something like this in the meantime?

python -m pip install --upgrade "pip!=20.0.0"

[hugovk]

And we don't need the workaround for this docs PR, and can possibly wait a bit for a new pip release before needing it. Merging, thanks!