Update tox.ini and CI for pytest 9 support

[paduszyk]

This PR updates our test matrix to add support for pytest 9 and improves the organization of test environments for different Python and pytest versions. The main changes focus on expanding and clarifying our CI coverage in both the GitHub Actions workflow and the tox.ini configuration.

Updates to CI test matrix and tox environments:

Test matrix improvements:

• Updated the test job in .github/workflows/test.yml to run tests using pytest 9 across all supported Python versions (3.10-3.14 and pypy3.10), and adjusted the tox_env values accordingly.
• Expanded the test-legacy-pytest job to include pytest 8 for all supported Python versions (3.9–3.14 and pypy3.10), in addition to the existing coverage for pytest 5-7 on Python 3.9.
• Fixed the Python version setup in the workflow to use the correct version from the matrix for each job, instead of hardcoding Python 3.9.

Tox configuration updates:

• Updated the envlist in tox.ini to add new environments for pytest 9 (py{310,311,312,313,314,py3}-pytest9) and to clarify pytest 8 coverage across all relevant Python versions.
• Added a dependency specification for pytest 9 in the deps section of tox.ini.