Drupal 10 requires Guzzle 7
It doesn't seem like there is any major change but I'm not super-familiar with everywhere civi uses guzzle: https://github.com/guzzle/guzzle/blob/master/UPGRADING.md#other-backwards-compatibility-breaking-changes
The main thing at the moment is you need to do something like composer require guzzlehttp/guzzle:"6.3.0 as 7.4.0" just to get the civi files downloaded, but that's obviously not the right thing to do.