Major version updates

[roelofr]

I wanted to update my Laravel's Guzzle installation, but that got blocked by this package.

So, when I tried updating it, I couldn't get any test to pass.

To avoid letting this project completely run into the ground, I've done a quick major overhaul.

• Switch from enum package to first-class Enums (hence the 8.1+ requirement)
• Update PHPunit from v6 to v9
• Update php-cs-fixer to a new version, and apply fixes for 8.1 compat
• Replace references of iZettle with Zettle by PayPal, and replace OAuth URLs

It's quite a big update, but all tests still seem green after some minor tweaks, so it should work pretty nicely.

Maybe this could be v1?

[LauLaman]

Hi @roelofr Sorry for the delay.. im working on projects like these in my free time.

[roelofr]

No worries, so do I