feat: update guzzle to version 7

[c0nst4ntin]

As raised in Issue Upgrade to Guzzle 7.x #170 Guzzle 7 is currently the most requested feature by the community.

This Pull-Request combines and improves changes from:

• Support GuzzleHTTP 7.x #184
• Guzzle ^7.0 & PHPUnit ^9.0 #171
• Updates Guzzle and PHP minimum version (7.1 is EOL) #168

During local testing it was possible to support both Guzzle six and seven.

[c0nst4ntin]

@gsteel Do you see any problem with releasing this as Version 5.3.0?

Should I drop ^6.3 from the composer.json and update the lock file again? Or keep it as it is?

[gsteel]

According to Travis, Guzzle was installed at ^7.0 - if you can't test ^6 in CI, how do we know it works with this patch? I'd just drop ^6, release a minor and be done with it. You will have to update the lockfile, yes.

[c0nst4ntin]

According to Travis, Guzzle was installed at ^7.0 - if you can't test ^6 in CI, how do we know it works with this patch? I'd just drop ^6, release a minor and be done with it. You will have to update the lockfile, yes.

Dropped support for Guzzle 6 in 93eaf2e

[gsteel]

The doc block additions duplicate the method signatures, but otherwise LGTM 👍 - The @final suggestion will pay dividends in future - I'm a final by default sort of person, but it's just opinion. My argument is that it's easy to relax this stuff in future if there's good reason, but hard to tighten things up without breaking BC

[c0nst4ntin]

I added your suggestion. Should I merge and release now?

[gsteel]

I added your suggestion. Should I merge and release now?

You're the boss 😉