Zttp is locked to an old Guzzle

[edgrosvenor]

I don't know how enthusiastic Adam is about the prospect of updating zttp to use Guzzle 7. Since you actually only use Zttp in one isolated method and everything could very easily be done using Guzzle or even Curl instead, does it maybe make sense to drop Zttp as a dependency and just reach down a level?

I only noticed this because I was going to use this package as a shortcut on a Laravel 8 project. I could go any one of a dozen different ways to solve this problem. But I figured I'd bring this to your attention to see if you'd like to do something here. If so I'll happily send a PR ripping out Zttp and replacing it with Guzzle (requiring 6 or 7).