devel/py-lxml: update to 6.0.4

Changelog: https://lxml.de/6.0/changes-6.0.4.html

For tests, copy built extension module artefacts into src instead
of building them again, and add cssselect TEST_DEPENDS

(cherry picked from commit 8ae5bc33a128b397a9a4cd1789d1a53e8471f2e6)