PHPUnit 9.3 released a new schema for the phpunit.xml configuration file. I had to migrate the file to the new format in order to avoid the warning. PHPUnit Versions older than 9.3 have to use the phpunit.xml.legacy configuration file, because the new format is unknown for them.
I also had to update some tests, because the at() matcher deprecated in PHPUnit 9.3 and will be removed in PHPUnit 10.
Together with the changes from #391 by @clue it's possible to run this code with PHP 8 without any warnings in your output 🎉
$ docker run -it --rm -v `pwd`:/data --workdir=/data php:8.0.0beta2-cli php vendor/bin/phpunit
PHPUnit 9.3.8 by Sebastian Bergmann and contributors.
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Time: 00:16.651, Memory: 20.00 MB
OK (673 tests, 1604 assertions)For further details concerning this pull request look into graphp/graphviz#46.
This pull request builds on top of reactphp/http#364.