Override writable method on OutStream

[takluyver]

Closes jupyter/notebook#3349

[Carreau]

Appveyor test seem to be an error on appveyor side. I can't seem to have the ability to restart the tests.

[takluyver]

I do have the ability, but they still fail. It looks like pip is failing to update itself.

[minrk]

We ran into the pip issue elsewhere. It's the fact that pip.exe is open prevents upgrade. You can upgrade pip with python -m pip install --upgrade pip on Windows.

[takluyver]

Ah, that makes sense. #317 reinstates the pip upgrade.