Support python-slugify 9

[BenA-SA]

Both packages pin python-slugify>=6.0.0,<9.0.0, so they can't be installed alongside python-slugify 9:

$ pip install "pytest-playwright==0.9.0" "python-slugify==9.1.2"
ERROR: Cannot install pytest-playwright==0.9.0 and python-slugify==9.1.2 because these package versions have conflicting dependencies.
    pytest-playwright 0.9.0 depends on python-slugify<9.0.0 and >=6.0.0

Widening to <10.0.0 looks safe: 9.x keeps the legacy output as its default, and slugify(nodeid) gives identical output under 8.0.4 and 9.1.2. Same situation as #159.