test: fix deprecation warning about path by using suggested Pathlib

[shuuji3]

I saw the following deprecation warning when running the tests on local:

> pipx run nox -s test
nox > Running session test
nox > Re-using existing virtual environment at .nox/test.
nox > python -m pip install -e .
nox > python -m pip install -r dev-requirements.txt
nox > pytest
=============================================================================== test session starts ===============================================================================
platform darwin -- Python 3.11.5, pytest-7.1.3, pluggy-1.3.0
rootdir: /Users/shuuji3/dev/sphinxext-opengraph
collected 37 items

tests/test_options.py ..........s..........................                                                                                                                 [100%]

================================================================================ warnings summary =================================================================================
tests/conftest.py:3
  /Users/shuuji3/dev/sphinxext-opengraph/tests/conftest.py:3: RemovedInSphinx90Warning: 'sphinx.testing.path' is deprecated. Use 'os.path' or 'pathlib' instead.
    from sphinx.testing.path import path

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
==================================================================== 36 passed, 1 skipped, 1 warning in 1.67s =====================================================================
nox > Session test was successful.

This was introduced by the recent changes, so I don't think it is an urgent one.

ref. Changelog — Sphinx documentation - https://www.sphinx-doc.org/en/master/changes.html#release-7-2-0-released-aug-17-2023

[shuuji3]

OK, it seems that this change affects many places of tests run on the GitHub Actions. This needs to be investigated.