ask for sudo password if necessary

[bethebunny]

Use kernel.getpass() to ask for the sudo password if we detect the system asked for it. This is a bit opening up a can of worms of making some specific prompts available to interact with, but I think this one is specialized and useful enough to deserve it.

https://github.com/takluyver/bash_kernel/issues/83

[kdm9]

Hi @bethebunny

I've recently taken on co-maintaining bash_kernel, and I'm currently closing all old/stale issues and PRs.

This PR looks really useful, so thanks! In principle, I'm happy to merge, but can you please rebase this on the current main branch (version 0.8), and check that this still functions as you wish. I can then review it in detail and merge it unless @takluyver objects.