Guzzle v7 Support

[MannikJ]

I'm not sure how much effort it would actually take, but are there plans to support Guzzle v7 in the near future?

[superDuperCyberTechno]

Nothing in the upgrade docs seems to conflict with anything that Zttp.php does.

I've forked the repo, upgraded the Guzzle dependency and made some preliminary tests. Seems to be working just fine...

Pull request here.