Do not extend Guzzle\Client

[tomboe311]

Extending Guzzle\Client is deprecated since Guzzle 7.1 and the class is annotated as @Final. The class will be actually final in Guzzle 8.0.

See guzzle issue#2521

[tacman]

On a related note, what about switching to the Symfony HttpClient? Or simply requiring that the application implement a psr-7 interface, and not providing a specific implementation of one?