Support for PHP8

[tmp-hallenser]

PHP8 has just been released. According to composer.json, this repo requires "php": "^7.1". Any plans to make it work with PHP8?

[dbu]

guzzle 6 does not work with php 8, so this would not help (they used the >7.1 version constraint, but that was a mistake).

for php 8, you should use guzzle 7 (which implements psr-18 natively) or if you need the httplug async interface, use guzzle7-adapter with guzzle 7.