Add -force argument for ungraceful disconnect

[Sjors]

I found it useful to test a less graceful disconnection than what shutdown() produces.

This PR adds a -force argument which calls close() instead. This results in a Bad file descriptor (9) error, although ideally I'd like to it produce a Connection reset by peer (104) error.

Builds on top of Python 3 support in #1.

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[Sjors]

@googlebot I signed it!

[googlebot]

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[Sjors]

@caruccio looks like the Google bot is confused because I'm building on top of your PR, even you did consent.

[caruccio]

So what can we do now? Maybe a fresh new pr will do the job? Fell free to handle at your will.

[Sjors]

@caruccio the bot says you just need to say @googlebot I consent.. Alternatively if the maintainer (@geffner) merges your PR, I can rebase on master.

[geffner]

Hi guys, thank you for your contributions! FYI, I'm no longer at Google and am no longer maintaining this codebase.