Allow Symfony 8

[W0rma]

Symfony 8 is going to be released at the end of November.

According to https://github.com/symfony/symfony/blob/8.0/UPGRADE-8.0.md symfony/css-selector is not affected by any BC breaking changes.

I ran the test suite locally with symfony/css-selector:8 and the tests were still green.

It would be great if we could get a new release after this PR was merged.