Update PHPUnit dependency to >=7.0

[mykeels]

Thanks for this!

I'm using this package on a project with a PHPUnit dependency of ^7.0, and composer require fails because this package's PHPUnit dependency is marked as ^6.0.

I have verified that using ^7.0 works in this fork, and it would be helpful to change the version referenced in this project to ^7.0 or >=6.0.