Adopt pytest's strict mode

[tpvasconcelos]

Closes #240

Summary

Went through pytest's Good Integration Practices guide and audited the project against it. Most recommendations were already in place:

• ✅ src layout with tests outside application code
• ✅ --import-mode=importlib (so the test package name / duplicate-basename concerns don't apply — e.g. the two test_init.py modules collect fine without __init__.py files)
• ✅ tox testing against the built wheel
• ✅ flake8-pytest-style checks (via ruff's select = ["ALL"])

The one remaining gap was pytest's strict mode, added in pytest 9.0. This PR:

• Enables strict = true in pytest.ini, superseding the previous --strict-markers flag and xfail_strict ini option. This also enables strict_config and strict_parametrization_ids, and will automatically pick up new strictness options as they are added to pytest (consistent with the project's approach of, e.g., ruff's select = ["ALL"] and filterwarnings = error). If you'd rather not auto-adopt future options, the alternative is to enable the four current strict_* options individually.
• Adds a pytest>=9 floor to requirements/tests.txt (pytest 9 supports Python >=3.10, matching this project's minimum).
• Fixes duplicate parametrization IDs surfaced by strict_parametrization_ids in test_hist.py and test_kde.py (np.inf/float("inf") and np.nan/float("nan") generate identical IDs) by adding explicit unique ids=.
• Fixes test_utils.py passing an itertools.product iterator to @pytest.mark.parametrize, which pytest 9.1 deprecates for removal in pytest 10. Note this was already breaking test collection with pytest 9.1.x on main (the PytestRemovedIn10Warning is promoted to an error by filterwarnings = error), independently of this PR.
• Refreshes the pinned 8.3.x doc links in pytest.ini comments to stable.

Test plan

• pytest tests/unit --doctest-modules src — 237 passed, 3 xfailed
• pytest tests/e2e — 5 passed (1 pre-existing local-only failure: test_paths_exist asserts the repo dir is named ridgeplot, which doesn't hold in a worktree checkout; passes on CI)
• pytest tests/cicd_utils — 24 passed
• pre-commit hooks (incl. ruff + ruff-format) pass
• pyright — 0 errors

---

📚 Documentation preview 📚: https://ridgeplot--387.org.readthedocs.build/en/387/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9b464c8) to head (6c79423).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #387   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          657       657           
  Branches        85        85           
=========================================
  Hits           657       657           

Flag	Coverage Δ
combined-src	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.