Support PHP ^8.0 in composer constraints

[theshaunwalker]

Hey @lucleroy

Not sure if you're around as its been a few years since your activity on Github, but before I forked this into another package wanted to throw up a PR incase you're around to click merge.

I want to use this library with PHP 8 projects, and as far as I can tell its fully compatible with PHP 8 so I've just added it as a constraint to the composer.json.

How I know its compatible:

• I cloned it locally in my PHP 8.1.2 and ran the PHPUnit tests and they passed
• Ran codesniffer to check PHP compatability (guide here https://odan.github.io/2020/12/22/php8-compatibility-check.html) and ran it against 8.0 and 8.1 compatability and it had no issues other than a warning about a debug_backtrace() call seems irrelevant as its to do with arguments and the call has no arguments. Plus its a warning from PHP 7.

Sooooo seems good to me? 😀

[lucleroy]

Thanks @theshaunwalker.