Support Guzzle 7

[ejunker]

I would be nice if this package could also work with Guzzle 7. ^6.3|^7.0

Guzzle 7 Upgrade Guide

[jspaetzel]

@jakubkulhan could you tag a new release with this change?

[jakobwildrain-si]

@jakubkulhan could you tag a new release with this change?

I second that. Would you, please, when you get a chance?