Extension breaks sites running Civi 5.69+

[MegaphoneJon]

This extension ships with Guzzle 6. CiviCRM 5.69+ ships with Guzzle 7. Because of namespace collisions, this causes crashes in Civi on many screens, as well as any CMS plugins that include Guzzle 7.

I'm experimenting with replacing the composer.json with the one in the mailchimp-api repo from Thinkshout and running composer upgrade, but it turns out I don't have a client actively using this extension who's on 5.69 yet.