Document local test setup and pytest commands

[yuefdev]

Fixes #430.

Add local test instructions using tox to create the environment and install test dependencies. Show how to pass pytest arguments for a test file, a name filter and skip reasons. Link to the existing CI configuration for supported Python versions rather than duplicating the version list.

Validation on Windows, Python 3.13.15:

• uvx --with tox-uv --python 3.13 tox -r -e py -- tests/test_number.py -k intcomma: 36 passed, 205 deselected. Used tox-uv after the local virtualenv/pip bootstrap failed with No module named pip.__main__.
• uvx prek run --files .github/CONTRIBUTING.md: passed.
• git diff --check: passed.

AI-assisted review follow-up using OpenCode and gpt-6-astra. Only the contributor guide changes.

[hugovk]

It's better to use tox locally, it handles the venv stuff. Don't repeat supported Python versions.

[yuefdev]

Updated the guide to use tox -e py and pass pytest arguments after --. Removed the repeated Python version requirement and linked to the CI matrix. Verified the focused tox invocation with tox-uv (36 tests passed); the contributor-guide pre-commit checks pass.