Update PHPUnit configuration schema for PHPUnit 9.3 and support PHP 8

[clue]

Refs clue/qdatastream#38 and graphp/graphviz#46

It's now possible to run this code with PHP 8 🎉

$ docker run -it --rm -v `pwd`:/data --workdir=/data php:8.0.0-cli php vendor/bin/phpunit
PHPUnit 9.5.0 by Sebastian Bergmann and contributors.

...................................................SSSSSSSSSS.... 65 / 91 ( 71%)
..........................                                        91 / 91 (100%)

Time: 00:01.156, Memory: 10.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 91, Assertions: 164, Skipped: 10.