chore: Fix deprecation warning for sphinx.testing.path

[Mr-Pepe]

Fixes

tests/conftest.py:10
  .../sphinx-autodoc-typehints/tests/conftest.py:10: RemovedInSphinx90Warning: 'sphinx.testing.path' is deprecated. Use 'os.path' or 'pathlib' instead.
    from sphinx.testing.path import path

[gaborbernat]

Please fix the CI

[Mr-Pepe]

os.path and pathlib are not drop-in replacements for sphinx.testing.path. Sphinx 7.2 dropped support for Python 3.8 and the last version that supports it (7.1) depends on specific functionality from sphinx.testing.path.

You can either drop support for Python 3.8 or close this PR and hope that Python 3.8 goes EOL before Sphinx 9.0 comes out and removes sphinx.testing.path 🤷