Fix warning about using srcdir as a string

[AWhetter]

Running the tests outputs the following warning:

tests/test_integration.py::TestIntegration::test_template_overrides
  /home/ashley/workspace/sphinx-autoapi/.tox/py313/lib/python3.13/site-packages/autoapi/extension.py:106: RemovedInSphinx80Warning: Sphinx 8 will drop support for representing paths as strings. Use "pathlib.Path" or "os.fspath" instead.
    elif app.srcdir != os.getcwd():

We should address this before Sphinx v8 is released and this warning becomes an error.